Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net earnings	$ 263,756	$ 200,300	$ 761,033	$ 501,345
Basic earnings (loss) per common share:
Net earnings (in dollars per basic share)	$ 1.83	$ 1.39	$ 5.29	$ 3.48
Weighted average shares outstanding - basic (in shares)	143,976,000	144,032,000	143,895,000	144,082,000
Diluted earnings (loss) per common share:
Net earnings (in dollars per diluted share)	$ 1.81	$ 1.38	$ 5.24	$ 3.45
Weighted average shares outstanding - diluted (in shares)	145,440,000	145,289,000	145,220,000	145,313,000
Reconciliation Of Share Amounts Used In Computing Earnings Per Share [Abstract]
Weighted average shares outstanding - basic (in shares)	143,976,000	144,032,000	143,895,000	144,082,000
Dilutive effect of assumed exercise of SAR's and vesting of performance shares (in shares)	1,464,000	1,257,000	1,325,000	1,231,000
Weighted average shares outstanding - diluted (in shares)	145,440,000	145,289,000	145,220,000	145,313,000
Antidilutive securities excluded from computation of earnings per share	1,000	36,000	57,000	95,000