Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Basic:
Net income	$ 129,565	$ 64,302	$ 219,492	$ 170,409
Participating securities' share in earnings (in dollars)	(2,131)	(1,293)	(3,543)	(3,589)
Net income applicable to common shareholders	127,434	63,009	215,949	166,820
Diluted:
Net income	129,565	64,302	219,492	170,409
Participating securities' share in earnings (in dollars)	(2,128)	(1,288)	(3,539)	(3,575)
Net income applicable to common shareholders	$ 127,437	$ 63,014	$ 215,953	$ 166,834
Shares:
Basic shares outstanding	84,942	83,984	84,942	83,984
Incremental shares from assumed exercise of stock options	112	335	101	353
Fully diluted common stock (in shares)	85,054	84,319	85,043	84,337
Excluded antidilutive securities (in shares)	100	249	156	259
Earnings per share to common shareholders
Basic (in dollars per share)	$ 1.50	$ 0.75	$ 2.54	$ 1.98
Diluted (in dollars per share)	$ 1.49	$ 0.74	$ 2.53	$ 1.97