Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Basic
Average common shares outstanding (in shares)		91,992,854	92,196,458	91,752,482	92,398,962
Net income		$ 386,733	$ 374,491	$ 929,673	$ 855,832
Basic net income per common share (in dollars per share)		$ 4.20	$ 4.06	$ 10.13	$ 9.26
Diluted
Average common shares outstanding (in shares)		91,992,854	92,196,458	91,752,482	92,398,962
Stock options and other contingently issuable shares (in shares)	[1]	2,218,617	1,698,414	2,109,359	1,864,371
Non-vested restricted stock grants (in shares)		560,336	538,203	559,349	513,054
Average common shares outstanding assuming dilution (in shares)		94,771,807	94,433,075	94,421,190	94,776,387
Net income		$ 386,733	$ 374,491	$ 929,673	$ 855,832
Diluted net income per common share (in dollars per share)		$ 4.08	$ 3.97	$ 9.85	$ 9.03
Average common shares outstanding, anti-dilutive (in shares)		32,633	34,473	63,310	31,615
Percent restricted shares representing outstanding shares (less than)						1.00%

[1] Stock options and other contingently issuable shares for the three and nine months ended September 30, 2016 excluded 32,633 and 63,310 shares, respectively, due to their anti-dilutive effect. Stock options and other contingently issuable shares for the three and nine months ended September 30, 2015 excluded 34,473 and 31,615 shares, respectively, due to their anti-dilutive effect .