Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of information used in computing basic and diluted earnings per share [Abstract]
Earnings from continuing operations	$ 296,448	$ 178,912	$ 164,058	$ 172,247	$ 161,162	$ 130,084	$ 118,290	$ 99,356	$ 811,665	$ 508,892	$ 595,881
Earnings from discontinued operations, net									0	0	273,948
Net earnings									$ 811,665	$ 508,892	$ 869,829
Basic earnings (loss) per common share:
Earnings from continuing operations (in dollars per basic share)	$ 1.90	$ 1.15	$ 1.05	$ 1.11	$ 1.04	$ 0.84	$ 0.76	$ 0.64	$ 5.21	$ 3.28	$ 3.78
Earnings from discontinued operations, net (in dollars per basic share)									0.00	0.00	1.74
Net earnings (in dollars per basic share)									$ 5.21	$ 3.28	$ 5.52
Weighted average shares outstanding - basic (in shares)									155,685,000	155,231,000	157,619,000
Diluted earnings (loss) per common share:
Earnings from continuing operations (in dollars per diluted share)	$ 1.88	$ 1.14	$ 1.04	$ 1.09	$ 1.03	$ 0.83	$ 0.76	$ 0.64	$ 5.15	$ 3.25	$ 3.74
Earnings (loss) from discontinued operations, net (in dollars per diluted share)									0.00	0.00	1.72
Net earnings (in dollars per diluted share)									$ 5.15	$ 3.25	$ 5.46
Weighted average shares outstanding - diluted (in shares)									157,744,000	156,636,000	159,172,000
Reconciliation Of Share Amounts Used In Computing Earnings Per Share [Abstract]
Weighted average shares outstanding - basic (in shares)									155,685,000	155,231,000	157,619,000
Dilutive effect of assumed exercise of employee stock options, SAR's and performance shares (in shares)									2,059,000	1,405,000	1,553,000
Weighted average shares outstanding - diluted (in shares)									157,744,000	156,636,000	159,172,000
Weighted average number of anti-dilutive potential common shares excluded from reconciliation calculations (in shares)									79,756	6,799	25,313