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Earnings Per Common Share Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net earnings	$ 56,300	$ 64,030	$ 38,493	$ 49,292	$ 40,491	$ 31,277	$ 94,455	$ 7,323	$ 208,115	$ 173,546	$ 8,492
Weighted-average shares outstanding									54,187	54,325	53,471
Effect of dilutive stock awards									1,567	1,630	1,071
Weighted-average dilutive shares									55,754	55,955	54,542
Basic earnings per common share	$ 1.04	$ 1.19	$ 0.72	$ 0.90	$ 0.74	$ 0.57	$ 1.74	$ 0.14	$ 3.84	$ 3.19	$ 0.16
Diluted earnings per common share	$ 1.00	$ 1.15	$ 0.70	$ 0.88	$ 0.72	$ 0.56	$ 1.70	$ 0.13	$ 3.73	$ 3.10	$ 0.16
Restricted share units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted earnings per common share									1	0	0
Performance share units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted earnings per common share									3	290	200
Stock options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted earnings per common share									949	1,109	1,498