reserved 8.5 million shares of common stock for issuance under the EIP and at December 31, 2021, we had 5.5 million shares available for issuance under the plan. This calculation of available shares reflects shares issued and estimated shares expected to be issued upon vesting of outstanding awards granted under the EIP, excluding estimated forfeitures expected to be returned to the plan.

Restricted Stock Units - We have granted restricted stock units to key employees that vest at the end of a three-year period and entitle the grantee to receive shares of our common stock. Restricted stock unit awards are measured at fair value as if they were vested and issued on the grant date and adjusted for estimated forfeitures. Restricted stock unit awards accrue dividend equivalents in the form of additional restricted stock units prior to vesting. Compensation expense is recognized on a straight-line basis over the vesting period of the award.

Performance Unit Awards - We have granted performance unit awards to key employees that vest at the end of a three-year period. Upon vesting, a holder of outstanding performance units is entitled to receive a number of shares of our common stock equal to a percentage (0% to 200%) of the performance units granted, based on our total shareholder return over the vesting period, compared with the total shareholder return of a peer group of other energy companies over the same period. Performance unit awards are measured at fair value on the grant date based on a Monte Carlo model and adjusted for estimated forfeitures. Performance unit awards accrue dividend equivalents in the form of additional performance units prior to vesting. Compensation expense is recognized on a straight-line basis over the vesting period of the award.

The EIP provides for the granting of nonstatutory stock options and stock bonus awards to non-employee directors, including performance unit awards and restricted stock unit awards. Under the EIP, awards may be granted by the Executive Compensation Committee at any time, until grants have been made for all shares authorized under the EIP. The maximum number of shares of common stock and cash-based awards that can be issued to a participant under the EIP during any year is limited to $0.8 million in value as of the grant date. No performance unit awards or restricted stock unit awards have been made to non-employee directors, and there are no options outstanding.

For all awards outstanding, we used a 3% forfeiture rate based on historical forfeitures under our share-based payment plans. We currently use treasury stock to satisfy our share-based payment obligations.

Compensation expense for our share-based payment plans was $54.1 million, $29.4 million and $46.5 million during 2021, 2020 and 2019, respectively, before related tax benefits of $14.4 million, $14.1 million and $31.7 million, respectively.

As of December 31, 2021, we had $19.3 million of total unrecognized compensation cost related to our nonvested restricted stock unit awards, which is expected to be recognized over a weighted-average period of 1.8 years. The following tables set forth activity and various statistics for our restricted stock unit awards:


	Number of Units	Weighted Average Price
Nonvested December 31, 2020	646,287	$	63.85
Granted	417,212	$	46.84
Released to participants	(242,765)	$	51.57
Forfeited	(40,797)	$	55.27
Nonvested December 31, 2021	779,937	$	59.02


	2021		2020		2019
Weighted-average grant date fair value (per share)	$	46.84	$	76.49	$	58.07
Fair value of units granted (thousands of dollars)	$	19,542	$	16,552	$	15,238
Grant date fair value of units vested (thousands of dollars)	$	12,519	$	11,204	$	10,691

As of December 31, 2021, we had $31.7 million of total unrecognized compensation cost related to the nonvested performance unit awards, which is expected to be recognized over a weighted-average period of 1.8 years. The following tables set forth activity and various statistics related to the performance unit awards and the assumptions used in the valuations at the respective grant dates:


	Number of Units	Weighted Average Price
Nonvested December 31, 2020	834,246	$	75.96
Granted	542,183	$	62.03
Released to participants	(311,907)	$	64.00
Forfeited	(87,937)	$	68.37
Nonvested December 31, 2021	976,585	$	72.73


	2021		2020		2019
Weighted-average grant date fair value (per share)	$	62.03	$	88.43	$	68.02
Fair value of units granted (thousands of dollars)	$	33,632	$	25,028	$	23,020
Grant date fair value of units vested (thousands of dollars)	$	19,962	$	17,722	$	15,018

We have reserved a total of 11.6 million shares of common stock for issuance under our Employee Stock Purchase Plan (the ESPP). Subject to certain exclusions, all employees are eligible to participate in the ESPP. Employees can choose to have up to 10% of their base pay withheld from each paycheck during the offering period to purchase our common stock, subject to terms and limitations of the plan. The purchase price of the stock is 85% of the lower of its grant date or exercise date market price. Approximately 69%, 68% and 62% of employees participated in the plan in 2021, 2020 and 2019, respectively. Under the plan, we sold 277,012 shares at a weighted average of $38.98 per share in 2021, 359,977 shares at a weighted average of $27.78 per share in 2020 and 171,590 shares at a weighted average of $51.24 per share in 2019.

Under our Employee Stock Award Program, we issue, for no monetary consideration, to all eligible employees one share of our common stock when the per-share closing price of our common stock on the NYSE is at or above each one-dollar increment above its previous high closing price. The total number of shares of our common stock available for issuance under this program is 900,000. Shares issued to employees under this program during 2020 and 2019 totaled 2,871 and 14,022, respectively. Compensation expense related to the Employee Stock Award Program was $0.2 million and $1.0 million for 2020 and 2019, respectively. No shares were issued to employees under this program in 2021. As of the date of this report, the next award will be issued when our common stock closes at or above $78.

Our Deferred Compensation Plan for Non-Employee Directors provides our non-employee directors the option to defer all or a portion of their compensation for their service on our Board of Directors. Under the plan, directors may elect either a cash deferral option or a phantom stock option. Under the cash deferral option, directors may elect to defer the receipt of all or a portion of their annual retainer fees, which will be credited with interest during the deferral period. Under the phantom stock option, directors may defer all or a portion of their annual retainer fees and receive such fees on a deferred basis in the form of shares of common stock under our EIP, which earn the equivalent of dividends declared on our common stock. Shares are distributed to non-employee directors at the fair market value of our common stock at the date of distribution.