Net Income Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 ParticipatingSecurities		Mar. 31, 2013
Basic
Average common shares outstanding (in shares)	98,833,210		101,961,059
Net income	$ 115,457		$ 116,185
Less net income allocated to unvested restricted shares	(523)		(639)
Net income allocated to common shares	114,934		115,546
Basic net income per common share (in dollars per share)	$ 1.16		$ 1.13
Diluted
Average common shares outstanding (in shares)	98,833,210		101,961,059
Stock options and other contingently issuable shares (in shares)	2,025,671	[1]	2,295,204	[1]
Average common shares outstanding assuming dilution (in shares)	100,858,881		104,256,263
Net income	115,457		116,185
Less net income allocated to unvested restricted shares assuming dilution	(515)		(628)
Net income allocated to common shares assuming dilution	$ 114,942		$ 115,557
Diluted net income per common share (in dollars per share)	$ 1.14		$ 1.11
Average common shares outstanding, anti-dilutive (in shares)	2,932		0
Classes of participating securities	2
Percent common shares representing outstanding shares	99.00%
Percent restricted shares representing outstanding shares	1.00%

[1] Stock options and other contingently issuable shares excluded 2,932 shares for the three months ended March 31, 2014. There were no options excluded due to their anti-dilutive effect for the three months ended March 31, 2013.