Net Income Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013 ParticipatingSecurities		Jun. 30, 2012
Basic
Average common shares outstanding (in shares)	101,665,737		101,446,643		101,813,398		101,758,496
Net income	$ 257,287		$ 227,813		$ 373,472		$ 328,029
Less net income allocated to unvested restricted shares	(1,639)		(1,908)		(2,278)		(2,689)
Net income allocated to common shares	255,648		225,905		371,194		325,340
Basic net income per common share (in dollars per share)	$ 2.51		$ 2.23		$ 3.65		$ 3.20
Diluted
Average common shares outstanding (in shares)	101,665,737		101,446,643		101,813,398		101,758,496
Stock options and other contingently issuable shares (in shares)	2,231,043	[1]	2,556,485	[1]	2,218,320	[1]	2,341,550	[1]
Average common shares outstanding assuming dilution (in shares)	103,896,780		104,003,128		104,031,718		104,100,046
Net income	257,287		227,813		373,472		328,029
Less net income allocated to unvested restricted shares assuming dilution	(1,607)		(1,865)		(2,235)		(2,633)
Net income allocated to common shares assuming dilution	$ 255,680		$ 225,948		$ 371,237		$ 325,396
Diluted net income per common share (in dollars per share)	$ 2.46		$ 2.17		$ 3.57		$ 3.13
Average common shares outstanding, anti-dilutive	0		0		0		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] There were no options excluded due to their anti-dilutive effect for the three and six months ended June 30, 2013 and 2012.