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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net income	$ 273,172	$ 128,470	$ 117,760	$ 83,300	$ 228,027	$ 107,769	$ 103,324	$ 54,971	$ 602,703	$ 494,090	$ 474,338
Less: earnings distributed to participating securities									(1,100)	(755)	(583)
Less: undistributed earnings allocated to participating securities									(3,622)	(2,448)	(2,668)
Numerator for basic earnings per share									597,981	490,887	471,087
Add back: undistributed earnings allocated to participating securities									3,622	2,448	2,668
Less: undistributed earnings reallocated to participating securities									(3,602)	(2,429)	(2,643)
Numerator for diluted earnings per share									$ 598,001	$ 490,906	$ 471,112
Denominator:
Basic shares outstanding (shares)	325,975	340,171	345,240	347,815	348,699	350,147	361,009	366,748	339,747	356,576	370,377
Effect of dilutive securities (shares)	1,834	2,250	2,759	2,662	3,047	3,225	3,232	3,362	2,376	3,217	3,725
Diluted shares outstanding (shares)	327,809	342,421	347,999	350,477	351,746	353,372	364,241	370,110	342,123	359,793	374,102
Earnings per share:
Basic (usd per share)	$ 0.83	$ 0.37	$ 0.34	$ 0.24	$ 0.65	$ 0.31	$ 0.28	$ 0.15	$ 1.76	$ 1.38	$ 1.27
Diluted (usd per share)	$ 0.83	$ 0.37	$ 0.34	$ 0.24	$ 0.64	$ 0.30	$ 0.28	$ 0.15	$ 1.75	$ 1.36	$ 1.26