Net Income Per Common 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
Basic
Average common shares outstanding (in shares)	92,988,118	91,992,854	92,793,275	91,752,482
Continuing operations	$ 316,606	$ 386,733	$ 916,409	$ 929,673
Discontinued operations	0	0	(41,540)	0
Net income	$ 316,606	$ 386,733	$ 874,869	$ 929,673
Basic net income per common share
Continuing operations (in dollars per share)	$ 3.40	$ 4.20	$ 9.88	$ 10.13
Discontinued operations (in dollars per share)	0.00	0.00	(0.45)	0.00
Net income per common share (in dollars per share)	$ 3.40	$ 4.20	$ 9.43	$ 10.13
Diluted
Average common shares outstanding (in shares)	92,988,118	91,992,854	92,793,275	91,752,482
Stock options and other contingently issuable shares (in shares)	2,106,912	2,218,617	1,942,845	2,109,359
Non-vested restricted stock grants (in shares)	112,854	560,336	81,549	559,349
Average common shares outstanding assuming dilution (in shares)	95,207,884	94,771,807	94,817,669	94,421,190
Diluted net income per common share
Continuing operations (in dollars per share)	$ 3.33	$ 4.08	$ 9.67	$ 9.85
Discontinued operations (in dollars per share)	0.00	0.00	(0.44)	0.00
Net income per common share (in dollars per share)	$ 3.33	$ 4.08	$ 9.23	$ 9.85
Average common shares outstanding, anti-dilutive (in shares)	26,859	32,633	26,859	63,310