Net Income Per Common Share (Details) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015 USD ($) $ / shares shares	Sep. 30, 2014 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) class $ / shares shares	Sep. 30, 2014 USD ($) $ / shares shares
Basic
Average common shares outstanding | shares		92,196,458	94,800,191	92,398,962	96,744,423
Net income		$ 374,491	$ 326,240	$ 855,832	$ 733,144
Less net income allocated to unvested restricted shares		(1,669)	(1,992)	(3,683)	(4,237)
Net income allocated to common shares		$ 372,822	$ 324,248	$ 852,149	$ 728,907
Basic net income per common share (in dollars per share) | $ / shares		$ 4.04	$ 3.42	$ 9.22	$ 7.53
Diluted
Average common shares outstanding | shares		92,196,458	94,800,191	92,398,962	96,744,423
Stock options and other contingently issuable shares | shares	[1]	1,698,414	1,913,852	1,864,371	1,926,576
Average common shares outstanding assuming dilution | shares		93,894,872	96,714,043	94,263,333	98,670,999
Net income		$ 374,491	$ 326,240	$ 855,832	$ 733,144
Less net income allocated to unvested restricted shares assuming dilution		(1,641)	(1,955)	(3,618)	(4,160)
Net income allocated to common shares assuming dilution		$ 372,850	$ 324,285	$ 852,214	$ 728,984
Diluted net income per common share (in dollars per share) | $ / shares		$ 3.97	$ 3.35	$ 9.04	$ 7.39
Average common shares outstanding, anti-dilutive | shares		34,473	0	31,615	15,885
Classes of participating securities | class				2
Percent common shares representing outstanding shares		99.00%		99.00%
Percent restricted shares representing outstanding shares		1.00%		1.00%

[1] Stock options and other contingently issuable shares excluded 34,473 and 31,615 shares due to their anti-dilutive effect for the three and nine months ended September 30, 2015. Stock options and other contingently issuable shares excluded 15,885 shares for the nine months ended September 30, 2014. There were no options excluded due to their anti-dilutive effect for the three months ended September 30, 2014.