Net Income Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 class		Mar. 31, 2014
Basic
Average common shares outstanding	92,740,059		98,833,210
Net income	$ 131,404		$ 115,457
Less net income allocated to unvested restricted shares	(475)		(523)
Net income allocated to common shares	130,929		114,934
Basic net income per common share (in dollars per share)	$ 1.41		$ 1.16
Diluted
Average common shares outstanding	92,740,059		98,833,210
Stock options and other contingently issuable shares	2,005,070	[1]	2,025,671	[1]
Average common shares outstanding assuming dilution	94,745,129		100,858,881
Net income	131,404		115,457
Less net income allocated to unvested restricted shares assuming dilution	(467)		(515)
Net income allocated to common shares assuming dilution	$ 130,937		$ 114,942
Diluted net income per common share (in dollars per share)	$ 1.38		$ 1.14
Average common shares outstanding, anti-dilutive (in shares)	12,414		2,932
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 12,414 and 2,932 shares due to their anti-dilutive effect for the three months ended March 31, 2015 and 2014, respectively.