Stock-based compensation expense for the years ended December 31, 2020, 2019 and 2018 was $43.2 million, $30.8 million and $33.1 million, respectively, and is included in general and administrative expense in the Consolidated Statement of Operations. The related income tax benefit for the years ended December 31, 2020, 2019 and 2018 was $10.0 million, $7.0 million and $7.6 million, respectively.

Restricted stock awards are granted from time to time to employees of the Company. The fair value of restricted stock grants is based on the closing stock price on the grant date. Restricted stock awards generally vest either at the end of a three year service period or on a graded or graduated vesting basis at each anniversary date over a three or four year service period.

For awards that vest at the end of the service period, expense is recognized ratably using a straight-line approach over the service period. Under the graded or graduated approach, the Company recognizes compensation cost ratably over the requisite service period, as applicable, for each separately vesting tranche as though the awards are, in substance, multiple awards. For most restricted stock awards, vesting is dependent upon the employees' continued service with the Company, with the exception of employment termination due to death, disability or retirement. If included in the grant award, the Company accelerates the vesting period for retirement-eligible employees for purposes of recognizing compensation expense in accordance with the vesting provisions of the Company's stock-based compensation programs.

The Company used an annual forfeiture rate assumption of five percent for purposes of recognizing stock-based compensation expense for restricted stock awards. The annual forfeiture rates were based on the Company's actual forfeiture history for this type of award to various employee groups.


	Year Ended December 31,
	2020			2019			2018
	Shares	Weighted- Average Grant Date Fair Value per Share		Shares	Weighted- Average Grant Date Fair Value per Share		Shares	Weighted- Average Grant Date Fair Value per Share
Outstanding at beginning of period	58,834	$	25.19	150,293	$	28.12	161,450	$	28.00
Granted	—	—		55,500	25.29		—	—
Vested	(6,334)	24.39		(143,959)	28.29		(7,157)	25.17
Forfeited	(2,000)	25.29		(3,000)	25.29		(4,000)	28.45
Outstanding at end of period(1)(2)	50,500	$	25.29	58,834	$	25.19	150,293	$	28.12

(1)As of December 31, 2020, the aggregate intrinsic value was $0.8 million and was calculated by multiplying the closing market price of the Company's stock on December 31, 2020 by the number of non-vested restricted stock awards outstanding.

Compensation expense recorded for all restricted stock awards for the years ended December 31, 2020, 2019 and 2018 was $0.4 million, $1.3 million and $2.8 million, respectively. Unamortized expense as of December 31, 2020 for all outstanding restricted stock awards was $0.7 million and will be recognized over the next 1.4 years.

Restricted stock units are granted from time to time to non-employee directors of the Company. The fair value of the restricted stock units is based on the closing stock price on the grant date. These units vest immediately and compensation expense is recorded immediately. Restricted stock units are issued when the director ceases to be a director of the Company.


	Year Ended December 31,
	2020			2019			2018
	Shares	Weighted- Average Grant Date Fair Value per Share		Shares	Weighted- Average Grant Date Fair Value per Share		Shares	Weighted- Average Grant Date Fair Value per Share
Outstanding at beginning of period	574,219	$	18.47	490,415	$	17.41	407,563	$	16.17
Granted and fully vested	130,065	15.88		83,804	24.70		82,852	23.47
Issued	—	—		—	—		—	—
Forfeited	—	—		—	—		—	—
Outstanding at end of period(1)(2)	704,284	$	17.99	574,219	$	18.47	490,415	$	17.41

(1)As of December 31, 2020, the aggregate intrinsic value was $11.5 million and was calculated by multiplying the closing market price of the Company's stock on December 31, 2020 by the number of outstanding restricted stock units.

Compensation expense recorded for all restricted stock units for the year ended December 31, 2020, 2019 and 2018 was $2.1 million, $2.1 million and $1.9 million, respectively, which reflects the total fair value of these units.

The Company grants three types of performance share awards: two based on performance conditions measured against the Company's internal performance metrics (Employee Performance Share Awards and Hybrid Performance Share Awards) and one based on market conditions measured based on the Company's performance relative to a predetermined peer group (TSR Performance Share Awards). The performance period for these awards commences on January 1 of the respective year in which the award was granted and extends over a three-year performance period. For all performance share awards, the Company used an annual forfeiture rate assumption ranging from zero percent to seven percent for purposes of recognizing stock-based compensation expense for its performance share awards.

The fair value of performance share award grants based on internal performance metrics is based on the closing stock price on the grant date. Each performance share award represents the right to receive up to 100 percent of the award in shares of common stock.

Employee Performance Share Awards. The Employee Performance Share Awards vest at the end of the three-year performance period and the performance metrics are set by the Company's Compensation Committee. For the awards granted in 2020, an employee will earn 100 percent of the award on the third anniversary, provided that the Company averages $100 million or more of operating cash flow during the three-year performance period. For awards granted in 2019 and 2018, an employee will earn one-third of the award for each of the three performance metrics. The three performance metrics are based on the Company's average production, average finding costs and average reserve replacement over a three-year performance period. Based on the Company's probability assessment at December 31, 2020, it is considered probable that all of the criteria for these awards will be met.


	Year Ended December 31,
	2020			2019			2018
	Shares	Weighted- Average Grant Date Fair Value per Share		Shares	Weighted- Average Grant Date Fair Value per Share		Shares	Weighted- Average Grant Date Fair Value per Share
Outstanding at beginning of period	1,259,287	$	23.64	1,280,021	$	22.22	1,095,970	$	23.31
Granted	722,500	15.60		526,730	24.95		531,670	23.25
Issued and fully vested	(334,640)	22.60		(388,370)	20.49		(315,970)	27.71
Forfeited	(37,023)	20.38		(159,094)	24.29		(31,649)	22.33
Outstanding at end of period	1,610,124	$	20.31	1,259,287	$	23.64	1,280,021	$	22.22

Hybrid Performance Share Awards. The Hybrid Performance Share Awards have a three-year graded performance period. The awards vest 25 percent on each of the first and second anniversary dates and 50 percent on the third anniversary provided that the Company has $100 million or more of operating cash flow for the year preceding the vesting date, as set by the Company's Compensation Committee. If the Company does not meet the performance metric for the applicable period, then the portion of the performance shares that would have been issued on that anniversary date will be forfeited. Based on the Company's probability assessment at December 31, 2020, it is considered probable that the criteria for these awards will be met.


	Year Ended December 31,
	2020			2019			2018
	Shares	Weighted- Average Grant Date Fair Value per Share		Shares	Weighted- Average Grant Date Fair Value per Share		Shares	Weighted- Average Grant Date Fair Value per Share
Outstanding at beginning of period	692,788	$	23.90	662,388	$	22.48	574,354	$	22.72
Granted	506,412	15.60		315,029	24.95		321,720	23.25
Issued and fully vested	(295,649)	23.40		(284,629)	21.78		(233,686)	24.12
Forfeited	—	—		—	—		—	—
Outstanding at end of period	903,551	$	19.41	692,788	$	23.90	662,388	$	22.48

These awards have both an equity and liability component, with the right to receive up to the first 100 percent of the award in shares of common stock and the right to receive up to an additional 100 percent of the value of the award in excess of the equity component in cash. The equity portion of these awards is valued on the grant date and is not marked to market, while the liability portion of the awards is valued as of the end of each reporting period on a mark-to-market basis. The Company calculates the fair value of the equity and liability portions of the awards using a Monte Carlo simulation model.

TSR Performance Share Awards. The TSR Performance Share Awards granted are earned, or not earned, based on the comparative performance of the Company's common stock measured against a predetermined group of companies in the Company's peer group over a three-year performance period.


	Year Ended December 31,
	2020			2019			2018
	Shares	Weighted- Average Grant Date Fair Value per Share(1)		Shares	Weighted- Average Grant Date Fair Value per Share(1)		Shares	Weighted- Average Grant Date Fair Value per Share(1)
Outstanding at beginning of period	1,428,634	$	20.17	1,299,868	$	19.47	1,109,708	$	19.23
Granted	862,180	13.79		536,673	20.63		482,581	19.92
Issued and fully vested	(891,961)	19.89		(407,907)	18.57		(292,421)	19.29
Forfeited	—	—		—	—		—	—
Outstanding at end of period	1,398,853	$	16.41	1,428,634	$	20.17	1,299,868	$	19.47

The current portion of the liability, included in accrued liabilities in the Consolidated Balance Sheet at December 31, 2019 was $6.1 million. There was no current liability recorded at December 31, 2020. The non-current portion of the liability for the TSR Performance Share Awards, included in other liabilities in the Consolidated Balance Sheet at December 31, 2020 and 2019, was $6.8 million and $4.1 million, respectively. The Company made cash payments during the years ended December 31, 2020, 2019 and 2018 of $14.0 million, $5.0 million and $3.3 million, respectively.


	Year Ended December 31,
	2020			2019			2018
Fair value per performance share award granted during the period	$	13.79		$	20.63		$	19.92
Assumptions
Stock price volatility	29.5		%	31.3		%	37.3		%
Risk free rate of return	1.4		%	2.5		%	2.4		%


	December 31,
	2020	2019	2018
Fair value per performance share award at the end of the period	$10.37 - $10.81	$6.18 - $14.80	$15.15 - $20.12
Assumptions
Stock price volatility	42.4% - 52.4%	29.8% - 30.4%	29.9% - 31.1%
Risk free rate of return	0.1%	1.6%	2.5% - 2.6%

The stock price volatility was calculated using historical closing stock price data for the Company for the period associated with the expected term through the grant date of each award. The risk free rate of return percentages are based on the continuously compounded equivalent of the U.S. Treasury within the expected term as measured on the grant date.

Compensation expense recorded for both the equity and liability components of all performance share awards for the years ended December 31, 2020, 2019 and 2018 was $39.6 million, $28.8 million and $30.6 million, respectively. Total unamortized compensation expense related to the equity component of performance shares at December 31, 2020 was $28.0 million and will be recognized over the next 2.2 years.

As of December 31, 2020, the aggregate intrinsic value for all performance share awards was $63.7 million and was calculated by multiplying the closing market price of the Company's stock on December 31, 2020 by the number of unvested performance share awards outstanding. As of December 31, 2020, the weighted average remaining contractual term of unvested performance share awards outstanding was approximately 1.4 years.

On December 31, 2020, the performance period ended for two types of performance share awards that were granted in 2018. For the Employee Performance Share Awards, the calculation of the three-year average of the three internal performance metrics was completed in the first quarter of 2021 and was certified by the Compensation Committee in February 2021. As the Company achieved the three performance metrics, 481,784 shares with a grant date fair value of $11.2 million were issued in February 2021. For the TSR Performance Share Awards, 482,581 shares with a grant date fair value of $9.6 million were issued in December 2020 based on the Company's ranking relative to a predetermined peer group. Cash payments associated with these awards in the amount of $7.9 million were also made in December 2020 due to the Company's ranking relative to the peer group. The calculation of the award payout was certified by the Compensation Committee on December 31, 2020.

As of December 31, 2020, 495,774 shares of the Company's common stock representing vested performance share awards were deferred into the deferred compensation plan. During 2020, no shares were sold out of the plan. During 2020, a decrease to the deferred compensation liability of $0.6 million was recognized, which represents the decrease in the closing price of the Company's shares held in the trust during the period. The decrease in compensation expense was included in general and administrative expense in the Consolidated Statement of Operations.