Net Income Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 ParticipatingSecurities	Mar. 31, 2011
Basic
Average common shares outstanding	102,070,349	104,991,014
Net income	$ 100,216	$ 68,316
Less net income allocated to unvested restricted shares	(782)	(670)
Net income allocated to common shares	99,434	67,646
Basic net income per common share	$ 0.97	$ 0.64
Diluted
Average common shares outstanding	102,070,349	104,991,014
Stock options and other contingently issuable shares	2,387,739	2,371,655
Average shares and equivalents outstanding - diluted	104,458,088	107,362,669
Net income	100,216	68,316
Less net income allocated to unvested restricted shares assuming dilution	(768)	(662)
Net income allocated to common shares assuming dilution	$ 99,448	$ 67,654
Diluted net income per common share	$ 0.95	$ 0.63
Net Income Per Common Share (Textual) [Abstract]
Average common shares outstanding, anti-dilutive	0	54,675
Classes of participating securities	2
Percent common shares representing outstanding shares	99.00%
Percent restricted shares representing outstanding shares	1.00%