Net Income Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014 ParticipatingSecurities		Jun. 30, 2013
Basic
Average common shares outstanding (in shares)	96,599,869		101,665,737		97,716,539		101,813,398
Net income	$ 291,447		$ 257,287		$ 406,904		$ 373,472
Less net income allocated to unvested restricted shares	(1,722)		(1,639)		(2,245)		(2,278)
Net income allocated to common shares	289,725		255,648		404,659		371,194
Basic net income per common share (in dollars per share)	$ 3.00		$ 2.51		$ 4.14		$ 3.65
Diluted
Average common shares outstanding (in shares)	96,599,869		101,665,737		97,716,539		101,813,398
Stock options and other contingently issuable shares (in shares)	1,942,040	[1]	2,231,043	[1]	1,972,018	[1]	2,218,320	[1]
Average common shares outstanding assuming dilution (in shares)	98,541,909		103,896,780		99,688,557		104,031,718
Net income	291,447		257,287		406,904		373,472
Less net income allocated to unvested restricted shares assuming dilution	(1,690)		(1,607)		(2,205)		(2,235)
Net income allocated to common shares assuming dilution	$ 289,757		$ 255,680		$ 404,699		$ 371,237
Diluted net income per common share (in dollars per share)	$ 2.94		$ 2.46		$ 4.06		$ 3.57
Average common shares outstanding, anti-dilutive (in shares)	0		0		2,932		0
Classes of participating securities					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 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 and six months ended June 30, 2013.