Net Income Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014 ParticipatingSecurities		Sep. 30, 2013
Basic
Average common shares outstanding	94,800,191		100,460,185		96,744,423		101,362,328
Net income	$ 326,240		$ 262,966		$ 733,144		$ 636,438
Less net income allocated to unvested restricted shares	(1,992)		(1,700)		(4,237)		(3,978)
Net income allocated to common shares	324,248		261,266		728,907		632,460
Basic net income per common share (in dollars per share)	$ 3.42		$ 2.60		$ 7.53		$ 6.24
Diluted
Average common shares outstanding	94,800,191		100,460,185		96,744,423		101,362,328
Stock options and other contingently issuable shares	1,913,852	[1]	2,162,329	[1]	1,926,576	[1]	2,189,214	[1]
Average common shares outstanding assuming dilution	96,714,043		102,622,514		98,670,999		103,551,542
Net income	326,240		262,966		733,144		636,438
Less net income allocated to unvested restricted shares assuming dilution	(1,955)		(1,667)		(4,160)		(3,902)
Net income allocated to common shares assuming dilution	$ 324,285		$ 261,299		$ 728,984		$ 632,536
Diluted net income per common share (in dollars per share)	$ 3.35		$ 2.55		$ 7.39		$ 6.11
Average common shares outstanding, anti-dilutive (in shares)	0		16,609		15,885		16,609
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 months ended September 30, 2014. Stock options and other contingently issuable shares excluded 15,885 shares for the nine months ended September 30, 2014. There were 16,609 options excluded due to their anti-dilutive effect for the three and nine months ended September 30, 2013.