In May of 2017 the Board of Directors approved an amendment to the shareholder approved stock incentive plan (the Plan) to increase the number of available shares. The revised Plan allows for the granting of up to 8.5 million total shares of equity awards to executives, employees and non-employee directors. Awards available for grant under the Plan include, but are not limited to, stock options, stock appreciation rights, performance shares and other stock awards. Shares issued under the Plan may be newly issued shares or reissued treasury shares.
Stock options are generally granted with a strike price equal to the fair market value of the stock on the date of grant and a life of 10 years. Stock options granted have a three-year vesting period. The strike price may be higher than the fair value of the stock on the date of the grant, but cannot be lower. Compensation expense is recorded on a straight-line basis over the vesting period beginning on the grant date. The compensation expense is based on the fair value of each option grant using the Black-Scholes option pricing model. Options granted to employees eligible for retirement under our retirement plan are fully expensed at the grant date.
Stock options are also awarded through the Key Employee Stock Investment Plan (KESIP) which allows certain employees, other than officers, to purchase shares of common stock on an installment basis up to an established credit limit. For every even block of 100 KESIP shares purchased by the employee 50 stock options are granted. The options granted through the KESIP program are considered awards under the Plan and are vested immediately. Compensation expense for stock options granted through the KESIP program is recorded based on the fair value of each option grant using the Black-Scholes option pricing model.
Performance shares are granted as target awards and are earned based on certain measures of our operating performance. A payout factor has been established ranging from 0 to 200 percent of the target award based on our actual performance during the three-year performance period. The fair value of the award is equal to the average market price, adjusted for the present value of dividends over the vesting period, of our stock on the grant date. Compensation expense is recorded ratably over the period beginning on the grant date until the shares become unrestricted and is based on the amount of the award that is expected to be earned under the plan formula, adjusted each reporting period based on current information.
Restricted common stock is awarded from time to time at no cost to certain employees. Participants are entitled to cash dividends and voting rights. Restrictions limit the sale or transfer of the shares during a defined period. Generally, one-third of the shares become vested and free from restrictions after two years and one-third of the shares issued become vested and free from restrictions each year thereafter on the anniversary of the grant date, provided the participant remains an employee. The fair value of the award is equal to the average market price of our stock on the grant date. Compensation expense is determined at the grant date and is recognized over the restriction period on a straight-line basis.
Employee compensation expense (net of estimated forfeitures) related to our share-based plans for the years ended December 31, 2018, 2017 and 2016, was approximately $52 million, $39 million and $31 million, respectively. In addition, non-employee director share-based compensation expense for the years ended December 31, 2018, 2017 and 2016, was approximately $1 million, $2 million and $1 million, respectively. Shares granted to non-employee directors vest immediately and have no restrictions or performance conditions. The excess tax benefit associated with our employee share-based plans for the years ended December 31, 2018, 2017 and 2016, was $2 million, $2 million and $1 million, respectively. The total unrecognized compensation expense (net of estimated forfeitures) related to nonvested awards for our employee share-based plans was approximately $46 million at December 31, 2018 and is expected to be recognized over a weighted-average period of less than two years.