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Common Stock Common Stock (Tables)
 12 Months Ended
Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Share-based Payment Arrangement, Cost by Plan [Table Text Block]
The following table provides a summary of the impact of our outstanding stock-based compensation plans on the results of operations for the periods presented:

Year Ended December 31,
202120202019
(in thousands)
General and administrative expense$21,830 $21,182 $22,754 
Lease operating expense1,193 1,018 1,083 
Total stock-based compensation expense$23,023 $22,200 $23,837 
Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity [Table Text Block]
The following table presents the changes in non-vested time-based RSUs to eligible employees, including executive officers, during the year ended December 31, 2021:

SharesWeighted Average Grant-Date Fair Value per Share
Non-vested at beginning of period1,150,970 $20.14 
Granted657,972 33.64 
Vested(547,985)24.86 
Forfeited(95,770)22.74 
Non-vested at end of period1,165,187 25.33 
Restricted Stock Awards, Market-Based, Valuation assumptions [Table Text Block]
Year Ended December 31,
202120202019
Expected term of award (in years)333
Risk-free interest rate0.2 %1.4 %2.5 %
Expected volatility84.6 %46.6 %41.4 %
Weighted average grant-date fair value per share$54.01 $33.52 $56.68 
Schedule of Nonvested Performance-based Units Activity [Table Text Block]
The following table presents the change in non-vested market-based awards during the year ended December 31, 2021:

SharesWeighted Average Grant-Date Fair Value per Share
Non-vested at December 31, 2020499,547 $38.66 
Granted207,655 54.01 
Vested(267,973)43.10 
Non-vested at December 31, 2021439,229 43.21 
Restricted Stock SRC - Market Based Awards [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted Stock Awards, Market-Based, Valuation assumptions [Table Text Block]