Net Income Per Common Share (Details) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) $ / shares shares	Dec. 31, 2015 class
Basic
Average common shares outstanding (in shares)		91,788,734	92,260,367	91,632,297	92,500,213
Net income | $		$ 378,064	$ 349,937	$ 542,940	$ 481,341
Basic net income per common share (in dollars per share) | $ / shares		$ 4.12	$ 3.79	$ 5.93	$ 5.20
Diluted
Average common shares outstanding (in shares)		91,788,734	92,260,367	91,632,297	92,500,213
Stock options and other contingently issuable shares (in shares)	[1]	2,318,192	1,864,328	2,114,844	1,926,978
Non-vested restricted stock grants (in shares)		562,825	467,362	558,856	500,479
Average common shares outstanding assuming dilution (in shares)		94,669,751	94,592,057	94,305,997	94,927,670
Net income | $		$ 378,064	$ 349,937	$ 542,940	$ 481,341
Diluted net income per common share (in dollars per share) | $ / shares		$ 3.99	$ 3.70	$ 5.76	$ 5.07
Average common shares outstanding, anti-dilutive (in shares)		16,013	21,243	47,273	21,243
Classes of participating securities | class						2
Percent restricted shares representing outstanding shares (less than)						1.00%

[1] Stock options and other contingently issuable shares excluded 16,013 and 47,273 shares due to their anti-dilutive effect for the three and six months ended June 30, 2016. Stock options and other contingently issuable shares excluded 21,243 shares due to their anti-dilutive effect for the three and six months ended June 30, 2015.