Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic
Average common shares outstanding (in shares)	93,099,714	92,988,118	93,121,900	92,793,275
Continuing operations	$ 354,027	$ 316,606	$ 1,007,758	$ 916,409
Discontinued operations	0	0	0	(41,540)
Net income	$ 354,027	$ 316,606	$ 1,007,758	$ 874,869
Basic net income per common share
Continuing operations (in dollars per share)	$ 3.80	$ 3.40	$ 10.82	$ 9.88
Discontinued operations (in dollars per share)	0.00	0.00	0.00	(0.45)
Net income per common share (in dollars per share)	$ 3.80	$ 3.40	$ 10.82	$ 9.43
Diluted
Average common shares outstanding (in shares)	93,099,714	92,988,118	93,121,900	92,793,275
Stock options and other contingently issuable shares (in shares)	1,982,841	2,106,912	1,990,622	1,942,845
Non-vested restricted stock grants (in shares)	52,702	112,854	58,246	81,549
Average common shares outstanding assuming dilution (in shares)	95,135,257	95,207,884	95,170,768	94,817,669
Diluted net income per common share
Continuing operations (in dollars per share)	$ 3.72	$ 3.33	$ 10.59	$ 9.67
Discontinued operations (in dollars per share)	0.00	0.00	0.00	(0.44)
Net income per common share (in dollars per share)	$ 3.72	$ 3.33	$ 10.59	$ 9.23
Stock options and other contingently issuable shares with anti-dilutive effects (in shares)	0	26,859	28,871	26,859