Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net earnings	$ 232,769	$ 176,279
Basic earnings (loss) per common share:
Net earnings (in dollars per basic share)	$ 1.62	$ 1.22
Weighted average shares outstanding - basic (in shares)	143,765,000	144,259,000
Diluted earnings (loss) per common share:
Net earnings (in dollars per diluted share)	$ 1.61	$ 1.21
Weighted average shares outstanding - diluted (in shares)	144,938,000	145,782,000
Reconciliation Of Share Amounts Used In Computing Earnings Per Share [Abstract]
Weighted average shares outstanding - basic (in shares)	143,765,000	144,259,000
Dilutive effect of assumed exercise of SAR's and vesting of performance shares (in shares)	1,173,000	1,523,000
Weighted average shares outstanding - diluted (in shares)	144,938,000	145,782,000
Antidilutive securities excluded from computation of earnings per share	88,000	65,000