Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net earnings	$ 124,766	$ 198,085	$ 301,045	$ 303,790
Basic earnings (loss) per common share:
Net earnings (in dollars per basic share)	$ 0.87	$ 1.36	$ 2.09	$ 2.09
Weighted average shares outstanding - basic (in shares)	143,955,000	145,366,000	144,107,000	145,227,000
Diluted earnings (loss) per common share:
Net earnings (in dollars per diluted share)	$ 0.86	$ 1.35	$ 2.07	$ 2.07
Weighted average shares outstanding - diluted (in shares)	144,995,000	147,179,000	145,359,000	147,041,000
Reconciliation Of Share Amounts Used In Computing Earnings Per Share [Abstract]
Weighted average shares outstanding - basic (in shares)	143,955,000	145,366,000	144,107,000	145,227,000
Dilutive effect of assumed exercise of SAR's and vesting of performance shares (in shares)	1,040,000	1,813,000	1,252,000	1,814,000
Weighted average shares outstanding - diluted (in shares)	144,995,000	147,179,000	145,359,000	147,041,000
Antidilutive securities excluded from computation of earnings per share	121,000	30,000	130,000	1,200