Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net earnings	$ 200,300	$ 206,006	$ 501,345	$ 509,796
Basic earnings (loss) per common share:
Net earnings (in dollars per basic share)	$ 1.39	$ 1.42	$ 3.48	$ 3.51
Weighted average shares outstanding - basic (in shares)	144,032,000	145,372,000	144,082,000	145,276,000
Diluted earnings (loss) per common share:
Net earnings (in dollars per diluted share)	$ 1.38	$ 1.40	$ 3.45	$ 3.47
Weighted average shares outstanding - diluted (in shares)	145,289,000	147,051,000	145,313,000	147,053,000
Reconciliation Of Share Amounts Used In Computing Earnings Per Share [Abstract]
Weighted average shares outstanding - basic (in shares)	144,032,000	145,372,000	144,082,000	145,276,000
Dilutive effect of assumed exercise of SAR's and vesting of performance shares (in shares)	1,257,000	1,679,000	1,231,000	1,777,000
Weighted average shares outstanding - diluted (in shares)	145,289,000	147,051,000	145,313,000	147,053,000
Antidilutive securities excluded from computation of earnings per share	36,000	28,000	95,000	9,000