Item 2. Management’s Discussion and Analysis of Financial Condition and Results of Operations - (continued)
the increase in interest payments due to the Merger-related debt refinancing; and
the timing of payments on accounts payable and certain accrued expenses, including incentive compensation based upon achieving certain performance goals for the year ended December 31, 2016. In accordance with most of our incentive plans, payments are typically made in the first six months of each fiscal year.
The principal source of our operating cash flow is the collection of accounts receivable from governmental payors, commercial payors and individuals. We bill for services as delivered, usually within a few days following the date the service is rendered for our ambulatory services segment and within 5 to 30 days following the date the service is rendered for our physician services segment. Generally, unpaid amounts that are 30 to 45 days past due are rebilled based on a standard set of procedures. If amounts remain uncollected after 60 days, we proceed with a series of late-notice notifications until amounts are either collected, contractually written off in accordance with contracted rates or determined to be uncollectible, typically after 90 to 240 days. Receivables determined to be uncollectible are written off and such amounts are applied to our estimate of allowance for bad debts as previously established in accordance with our policy for bad debt expense. The amount of actual write-offs of account balances for each of our subsidiaries is continuously compared to established allowances for bad debt to ensure that such allowances are adequate. At September 30, 2017, our physician services segment's net accounts receivable represented 69 days of revenue outstanding, which is an increase from 63 days outstanding at December 31, 2016 (which excluded the results from EHH). The increase in days is due to including the impact of EHH's physician services business, which historically has a longer billing and collection cycle as it is primarily focused on emergency department services, as well as the impact of the hurricanes occurring during the three months ended September 30, 2017, which contributed an increase of approximately 1.5 days. The increase is also due in part to recent acquisitions completed during the nine months ended September 30, 2017 and year ended December 31, 2016, as it is not unusual for us to experience delays in our ability to bill for procedures until certain administrative procedures are finalized. Also, we are currently migrating the billing of certain contracts to new billing systems, which we expect will improve efficiency in the billing cycle during 2017. As a result of this process, we are experiencing expected delays in our collections for our physician services. At both September 30, 2017 and December 31, 2016, our ambulatory services segment net accounts receivable represented 33 days of revenue outstanding.
As of September 30, 2017, we had insurance collateral of $102.5 million, which is comprised of restricted cash and available-for-sale securities that are restricted for the purpose of satisfying the obligations of our wholly owned captive insurance companies.
Investing activities. Net cash used in investing activities was $848.6 million for the nine months ended September 30, 2017 compared to $388.3 million for the nine months ended September 30, 2016. The change was primarily related to the funding of acquisitions and capital expenditures that occurred during the nine months ended September 30, 2017.
During the nine months ended September 30, 2017, we had total acquisition and capital expenditures of $832.9 million, which primarily included:
$562.0 million for the acquisition of physician practices;
$47.3 million for the acquisition of interests in surgery centers;
$78.0 million for the acquisitions in our medical transportation business; and
$138.5 million for new or replacement property.
Financing activities. Net cash provided by financing activities was $275.3 million for the nine months ended September 30, 2017 compared to $36.7 million for the nine months ended September 30, 2016. For the nine months ended September 30, 2017, we had net proceeds on long-term borrowings of $473.2 million, which included gross proceeds of $801.2 million and payments of $328.0 million. Our proceeds primarily resulted from $500.0 million of incremental borrowings under our Term Loan B 2023 and from borrowings from the ABL Facility. We used the incremental borrowings from the Term Loan B 2023 to fund acquisitions, to repay amounts outstanding under the ABL Facility, and to pay fees and expenses related to the financing.
During the nine months ended September 30, 2017, we repurchased approximately 135,532 shares of our common stock by withholding a portion of employee restricted stock that vested, with a value of approximately $9.4 million, to cover payroll withholding taxes in accordance with the restricted stock agreements.
On September 17, 2017, the Board authorized a stock repurchase program that authorizes us to repurchase up to $250 million of our common stock. The timing and amount of any shares repurchased will be determined based on our evaluation of market conditions and other factors. Repurchases will be made in accordance with the rules and regulations promulgated by the SEC and certain other legal requirements to which we may be subject. The program may be suspended or discontinued at any time, and has no time limit. As of September 30, 2017, we had made no repurchases under the stock repurchase program.
Our Company Preferred Stock paid dividends at an annual rate of 5.25% of the initial liquidation preference of $100 per share.