Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic
Average common shares outstanding (in shares)	92,926,421	92,841,148	93,132,993	92,695,853
Continuing operations	$ 403,604	$ 360,651	$ 653,731	$ 599,803
Discontinued operations	0	(41,540)	0	(41,540)
Net income	$ 403,604	$ 319,111	$ 653,731	$ 558,263
Basic net income per common share
Continuing operations (in dollars per share)	$ 4.34	$ 3.89	$ 7.02	$ 6.47
Discontinued operations (in dollars per share)	0.00	(0.45)	0.00	(0.45)
Net income per common share (in dollars per share)	$ 4.34	$ 3.44	$ 7.02	$ 6.02
Diluted
Average common shares outstanding (in shares)	92,926,421	92,841,148	93,132,993	92,695,853
Stock options and other contingently issuable shares (in shares)	1,903,442	2,055,422	2,064,944	1,935,690
Non-vested restricted stock grants (in shares)	54,324	72,066	61,019	65,896
Average common shares outstanding assuming dilution (in shares)	94,884,187	94,968,636	95,258,956	94,697,439
Diluted net income per common share
Continuing operations (in dollars per share)	$ 4.25	$ 3.80	$ 6.86	$ 6.34
Discontinued operations (in dollars per share)	0.00	(0.44)	0.00	(0.44)
Net income per common share (in dollars per share)	$ 4.25	$ 3.36	$ 6.86	$ 5.90
Stock options and other contingently issuable shares with anti-dilutive effects (in shares)	52,427	0	26,873	0