Net Income Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Basic
Average common shares outstanding	104,676,477	107,686,335	104,833,745	107,822,967
Net income	$ 179,115	$ 181,706	$ 247,431	$ 214,309
Less net income allocated to unvested restricted shares	(2,039)	(1,983)	(2,709)	(2,186)
Net income allocated to common shares	177,076	179,723	244,722	212,123
Basic net income per common share	$ 1.69	$ 1.67	$ 2.33	$ 1.97
Diluted
Average common shares outstanding	104,676,477	107,686,335	104,833,745	107,822,967
Stock options and other contingently issuable shares	2,199,984	2,146,317	2,270,280	1,637,652
Average common shares outstanding assuming dilution	106,876,461	109,832,652	107,104,025	109,460,619
Net income	179,115	181,706	247,431	214,309
Less net income allocated to unvested restricted shares assuming dilution	(2,004)	(1,950)	(2,666)	(2,154)
Net income allocated to common shares assuming dilution	$ 177,111	$ 179,756	$ 244,765	$ 212,155
Diluted net income per common share	$ 1.66	$ 1.64	$ 2.29	$ 1.94
Net Income Per Common Share (Textuals) [Abstract]
Average common shares outstanding, anti-dilutive	100,000	0	100,000	100,000
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%