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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income	$ 237,649	$ 289,535	$ 324,089	$ 170,751	$ 77,415	$ 177,539	$ 100,749	$ 91,518	$ 1,022,023	$ 447,221	$ 602,703
Less: earnings distributed to participating securities									(1,208)	(1,192)	(1,100)
Less: undistributed earnings allocated to participating securities									(9,984)	(3,380)	(3,622)
Numerator for basic earnings per share									1,010,831	442,649	597,981
Add back: undistributed earnings allocated to participating securities									9,984	3,380	3,622
Less: undistributed earnings reallocated to participating securities									(9,939)	(3,361)	(3,602)
Numerator for diluted earnings per share									$ 1,010,876	$ 442,668	$ 598,001
Denominator:
Basic shares outstanding (shares)	278,964	283,489	285,276	286,683	292,174	298,538	312,315	317,756	283,578	305,089	339,747
Effect of dilutive securities (shares)	1,248	1,183	1,378	1,343	1,318	1,690	1,565	2,329	1,287	1,725	2,376
Diluted shares outstanding (shares)	280,212	284,672	286,654	288,026	293,492	300,228	313,880	320,085	284,865	306,814	342,123
Earnings per share:
Basic (usd per share)	$ 0.84	$ 1.01	$ 1.12	$ 0.59	$ 0.26	$ 0.59	$ 0.32	$ 0.29	$ 3.56	$ 1.45	$ 1.76
Diluted (usd per share)	$ 0.84	$ 1.01	$ 1.12	$ 0.59	$ 0.26	$ 0.58	$ 0.32	$ 0.28	$ 3.55	$ 1.44	$ 1.75