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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ 335,798	$ 273,104	$ 241,041	$ 166,757	$ 237,649	$ 289,535	$ 324,089	$ 170,751	$ 1,016,700	$ 1,022,023	$ 447,221
Less: earnings distributed to participating securities									(1,228)	(1,208)	(1,192)
Less: undistributed earnings allocated to participating securities									(9,143)	(9,984)	(3,380)
Numerator for basic earnings per share									1,006,329	1,010,831	442,649
Add back: undistributed earnings allocated to participating securities									9,143	9,984	3,380
Less: undistributed earnings reallocated to participating securities									(9,117)	(9,939)	(3,361)
Numerator for diluted earnings per share									$ 1,006,355	$ 1,010,876	$ 442,668
Denominator:
Basic shares outstanding (shares)	270,843	272,992	276,652	277,637	278,964	283,489	285,276	286,683	274,495	283,578	305,089
Effect of dilutive securities (shares)	632	640	932	1,003	1,248	1,183	1,378	1,343	802	1,287	1,725
Diluted shares outstanding (shares)	271,475	273,632	277,584	278,640	280,212	284,672	286,654	288,026	275,297	284,865	306,814
Earnings per share:
Basic (usd per share)	$ 1.23	$ 0.99	$ 0.86	$ 0.59	$ 0.84	$ 1.01	$ 1.12	$ 0.59	$ 3.67	$ 3.56	$ 1.45
Diluted (usd per share)	$ 1.22	$ 0.99	$ 0.86	$ 0.59	$ 0.84	$ 1.01	$ 1.12	$ 0.59	$ 3.66	$ 3.55	$ 1.44