Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Reconciliation of information used in computing basic and diluted earnings per share [Abstract]
Net earnings	$ 172,247	$ 99,356
Basic earnings (loss) per common share:
Net earnings (in dollars per basic share)	$ 1.11	$ 0.64
Weighted average shares outstanding - basic (in shares)	155,540,000	155,064,000
Diluted earnings (loss) per common share:
Net earnings (in dollars per diluted share)	$ 1.09	$ 0.64
Weighted average shares outstanding - diluted (in shares)	157,399,000	156,161,000
Reconciliation Of Share Amounts Used In Computing Earnings Per Share [Abstract]
Weighted average shares outstanding - basic (in shares)	155,540,000	155,064,000
Dilutive effect of assumed exercise of employee stock options, SAR's and performance shares (in shares)	1,859,000	1,097,000
Weighted average shares outstanding - diluted (in shares)	157,399,000	156,161,000
Antidilutive securities excluded from computation of earnings per share	0	27,000