Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Basic:
Net income	$ 148,640	$ 129,565	$ 287,097	$ 219,492
Participating securities' share in earnings (in dollars)	(2,320)	(2,131)	(4,464)	(3,543)
Net income applicable to common shareholders	146,320	127,434	282,633	215,949
Diluted:
Net income	148,640	129,565	287,097	219,492
Participating securities' share in earnings (in dollars)	(2,316)	(2,128)	(4,457)	(3,539)
Net income applicable to common shareholders	$ 146,324	$ 127,437	$ 282,640	$ 215,953
Shares:
Basic shares outstanding	85,532	84,942	85,532	84,942
Incremental shares from assumed exercise of stock options	157	112	147	101
Fully diluted common stock (in shares)	85,689	85,054	85,679	85,043
Excluded antidilutive securities (in shares)		100	1	156
Earnings per share to common shareholders
Basic (in dollars per share)	$ 1.71	$ 1.50	$ 3.30	$ 2.54
Diluted (in dollars per share)	$ 1.70	$ 1.49	$ 3.29	$ 2.53