Net Income Per Common Share (Details) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2014 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) class $ / shares shares	Jun. 30, 2014 USD ($) $ / shares shares
Basic
Average common shares outstanding | shares		92,260,367	96,599,869	92,500,213	97,716,539
Net income		$ 349,937	$ 291,447	$ 481,341	$ 406,904
Less net income allocated to unvested restricted shares		(1,539)	(1,722)	(2,014)	(2,245)
Net income allocated to common shares		$ 348,398	$ 289,725	$ 479,327	$ 404,659
Basic net income per common share (in dollars per share) | $ / shares		$ 3.78	$ 3.00	$ 5.18	$ 4.14
Diluted
Average common shares outstanding | shares		92,260,367	96,599,869	92,500,213	97,716,539
Stock options and other contingently issuable shares | shares	[1]	1,864,328	1,942,040	1,926,978	1,972,018
Average common shares outstanding assuming dilution | shares		94,124,695	98,541,909	94,427,191	99,688,557
Net income		$ 349,937	$ 291,447	$ 481,341	$ 406,904
Less net income allocated to unvested restricted shares assuming dilution		(1,510)	(1,690)	(1,977)	(2,205)
Net income allocated to common shares assuming dilution		$ 348,427	$ 289,757	$ 479,364	$ 404,699
Diluted net income per common share (in dollars per share) | $ / shares		$ 3.70	$ 2.94	$ 5.08	$ 4.06
Average common shares outstanding, anti-dilutive (in shares) | shares		21,243	0	21,243	2,932
Classes of participating securities | class				2
Percent common shares representing outstanding shares		99.00%		99.00%
Percent restricted shares representing outstanding shares		1.00%		1.00%

[1] Stock options and other contingently issuable shares excluded 21,243 shares due to their anti-dilutive effect for the three and six months ended June 30, 2015. Stock options and other contingently issuable shares excluded 2,932 shares for the six months ended June 30, 2014. There were no options excluded due to their anti-dilutive effect for the three months ended June 30, 2014.