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Net Income Per Common Share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Mar. 31, 2021
Jun. 30, 2020
Mar. 31, 2020
Jun. 30, 2021
Jun. 30, 2020
Earnings Per Share [Abstract]            
Net income $ 302,556 $ 149,156 $ 33,586 $ 12,200 $ 451,712 $ 45,786
Computation of basic and diluted net income per common share            
Weighted average common shares outstanding during the period (for basic calculation) 39,442   39,312   39,399 39,238
Dilutive effect of other potential common shares 246   75   234 143
Weighted average common shares and potential common shares (for diluted calculation) 39,688   39,387   39,633 39,381
Net income per common share - Basic $ 7.67   $ 0.85   $ 11.47 $ 1.17
Net income per common share - Diluted $ 7.62   $ 0.85   $ 11.40 $ 1.16
Stock awards [Member]            
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]            
Antidilutive stock awards 0   100   200 100