Stock Incentive Plan. In May 2020, our stockholders approved an increase of 3.6 million shares authorized for issuances under the Amended and Restated 2005 Stock Incentive Plan (the “2005 Plan”), from 21.4 million shares to 25.0 million shares. Shares reserved under the 2005 Plan can be granted to officers and other key employees of our company and its subsidiaries and to non-employee directors of our company in the form of stock options, stock appreciation rights, performance unit awards, restricted stock awards, or stock bonus awards. Shares granted under the 2005 Plan in the form of a performance unit award, restricted stock award, or stock bonus award are counted toward the aggregate number of shares of common stock available for issuance under the 2005 Plan as two shares for every one share granted or issued in payment of such award. As of December 31, 2020, 5.8 million shares were available for issuance, with 5.5 million shares available for grant.

Restricted Stock. We generally issue new shares (versus treasury shares) to fulfill restricted stock award grants. Restricted stock awards are granted at no cost to the recipient. Historically, our restricted stock awards have vested annually primarily over two to four years with no restrictions other than the passage of time (i.e., the shares are released upon calendar vesting with no further restrictions) (or “Time-Based Awards”). Unvested Time-Based Awards are typically forfeited and cancelled upon termination of employment with our company. Certain Time-Based Awards become fully vested (i.e., vesting accelerates) upon a change in control, as defined, and the subsequent involuntary termination of employment, or death. The fair value of the Time-Based Awards (determined by using the closing market price of our common stock on the grant date) is charged to expense on a straight-line basis over the requisite service period for the entire award.

We also issue restricted stock shares to key members of management that vest upon meeting pre-established financial performance objectives (“Performance-Based Awards”). The structure of the performance goals for the Performance-Based Awards has been approved by our stockholders. Certain Performance-Based Awards become fully vested (i.e., vesting accelerates) upon a change in control, as defined, and the subsequent involuntary termination of employment. The fair value of the Performance-Based Awards (determined by using the closing market price of our common stock on the grant date) is charged to expense on a straight-line basis over a two-year performance period.

During 2020, market-based awards for 0.1 million restricted common stock shares were granted to certain members of executive management which vest upon meeting pre-established share price targets over a four-year period ("Market-Based Awards"). The Market-Based Awards become fully vested upon a change in control, as defined, and the subsequent involuntary termination of employment. The fair value of the Market-Based Awards (determined using a Monte Carlo valuation method), is charged to expense over the estimated service period for each separately vesting portion of the award as if the award is, in-substance, multiple awards.

A summary of our unvested restricted stock activity during 2020 is as follows (shares in thousands):


	2020
	Shares			Weighted- Average Grant Date Fair Value
Unvested awards, beginning		1,117		$	42.60
Awards granted		695			40.86
Awards forfeited/cancelled		(64	)		40.53
Awards vested		(707	)		43.15
Unvested awards, ending		1,041		$	41.31

The weighted-average grant date fair value per share of restricted stock shares granted during 2020, 2019, and 2018 was $40.86, $41.69, and $45.57, respectively. The total market value of restricted stock shares vesting during 2020, 2019, and 2018 was $32.8 million, $17.0 million, and $22.7 million, respectively.

1996 Employee Stock Purchase Plan. As of December 31, 2020, we have an employee stock purchase plan whereby 1.7 million shares of our common stock have been reserved for sale to our U.S. employees through payroll deductions. The price for shares purchased under the plan is 85% of market value on the last day of the purchase period. Purchases are made at the end of each month. During 2020, 2019, and 2018, 68,552 shares, 54,949 shares, and 68,902 shares, respectively, were purchased under the plan for $2.5 million ($32.20 to $42.35 per share), $2.3 million ($30.76 to $48.99 per share), and $2.4 million ($27.00 to $39.68 per share), respectively. As of December 31, 2020, 153,708 shares remain eligible for purchase under the plan.

Stock-Based Compensation Expense. We recorded stock-based compensation expense of $25.2 million, $19.9 million, and $19.4 million, respectively, for 2020, 2019, and 2018. As of December 31, 2020, there was $26.8 million of total compensation cost related to unvested awards not yet recognized. This amount, excluding the impact of forfeitures, is expected to be recognized over a weighted-average period of 2.3 years.

We recorded a deferred income tax benefit related to stock-based compensation expense during 2020, 2019, and 2018, of $5.8 million, $4.3 million, and $4.4 million, respectively. The actual income tax benefit realized for the tax deductions from the vesting of restricted stock for 2020, 2019, and 2018, totaled $4.0 million, $3.9 million, and $5.3 million, respectively.

Modifications to Stock-Based Awards. In August 2020, we entered into a Separation Agreement (the “Separation Agreement”), with our then-current President and Chief Executive Officer (“CEO”) which included a provision that accelerated the vesting of approximately 198,000 shares of unvested restricted stock on December 30, 2020. This modification resulted in a reversal of stock-based compensation expense in the third quarter of 2020 of $2.7 million. The fair value of the modified award of $8.4 million was recognized ratably from the date of modification through the resignation date, all of which was recognized in 2020.