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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				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Common Share
Net earnings	$ 38,493	$ 49,292	$ 94,455	$ 7,323	$ 87,785	$ 101,778
Weighted-average shares outstanding	53,800		54,153		54,287	54,042
Effect of dilutive stock awards	1,424		1,553		1,401	1,559
Weighted-average dilutive shares	55,224		55,706		55,688	55,601
Basic earnings per common share	$ 0.72		$ 1.74		$ 1.62	$ 1.88
Diluted earnings per common share	$ 0.70		$ 1.70		$ 1.58	$ 1.83
Retained Earnings [Member]
Earnings Per Common Share
Net earnings	$ 38,493	$ 49,292	$ 94,455	$ 7,323	$ 87,785	$ 101,778
Restricted share units [Member]
Earnings Per Common Share
Anti-dilutive securities	145		0		145	0
Performance-based restricted stock units [Member]
Earnings Per Common Share
Anti-dilutive securities	519		263		519	263
Stock options [Member]
Earnings Per Common Share
Anti-dilutive securities	1,349		1,107		1,340	1,123