Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reconciliation of information used in computing basic and diluted earnings per share [Abstract]
Net earnings	$ 178,912	$ 130,084	$ 515,217	$ 347,730
Basic earnings (loss) per common share:
Net earnings (in dollars per basic share)	$ 1.15	$ 0.84	$ 3.31	$ 2.24
Weighted average shares outstanding - basic (in shares)	155,757,000	155,300,000	155,668,000	155,182,000
Diluted earnings (loss) per common share:
Net earnings (in dollars per diluted share)	$ 1.14	$ 0.83	$ 3.27	$ 2.22
Weighted average shares outstanding - diluted (in shares)	157,555,000	156,798,000	157,565,000	156,562,000
Reconciliation Of Share Amounts Used In Computing Earnings Per Share [Abstract]
Weighted average shares outstanding - basic (in shares)	155,757,000	155,300,000	155,668,000	155,182,000
Dilutive effect of assumed exercise of SAR's and vesting of performance shares (in shares)	1,798,000	1,498,000	1,897,000	1,380,000
Weighted average shares outstanding - diluted (in shares)	157,555,000	156,798,000	157,565,000	156,562,000
Antidilutive securities excluded from computation of earnings per share	87,000	21,000	37,000	31,000