Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 77,415	$ 177,539	$ 100,749	$ 91,518	$ 273,172	$ 128,470	$ 117,760	$ 83,300	$ 447,221	$ 602,703	$ 494,090
Less: earnings distributed to participating securities									(1,192)	(1,100)	(755)
Less: undistributed earnings allocated to participating securities									(3,380)	(3,622)	(2,448)
Numerator for basic earnings per share									442,649	597,981	490,887
Add back: undistributed earnings allocated to participating securities									3,380	3,622	2,448
Less: undistributed earnings reallocated to participating securities									(3,361)	(3,602)	(2,429)
Numerator for diluted earnings per share									$ 442,668	$ 598,001	$ 490,906
Denominator:
Basic shares outstanding (shares)	292,174	298,538	312,315	317,756	325,975	340,171	345,240	347,815	305,089	339,747	356,576
Effect of dilutive securities (shares)	1,318	1,690	1,565	2,329	1,834	2,250	2,759	2,662	1,725	2,376	3,217
Diluted shares outstanding (shares)	293,492	300,228	313,880	320,085	327,809	342,421	347,999	350,477	306,814	342,123	359,793
Earnings per share:
Basic (usd per share)	$ 0.26	$ 0.59	$ 0.32	$ 0.29	$ 0.83	$ 0.37	$ 0.34	$ 0.24	$ 1.45	$ 1.76	$ 1.38
Diluted (usd per share)	$ 0.26	$ 0.58	$ 0.32	$ 0.28	$ 0.83	$ 0.37	$ 0.34	$ 0.24	$ 1.44	$ 1.75	$ 1.36