Basis of Presentation (Earnings per share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income	$ 324,089	$ 100,749	$ 494,841	$ 192,268
Less: earnings distributed to participating securities	(300)	(300)	(595)	(605)
Less: undistributed earnings allocated to participating securities	(3,284)	(772)	(2,584)	(1,330)
Numerator for basic earnings per share	320,505	99,677	491,662	190,333
Add back: undistributed earnings allocated to participating securities	3,284	772	2,584	1,330
Less: undistributed earnings reallocated to participating securities	(3,268)	(768)	(2,575)	(1,322)
Numerator for diluted earnings per share	$ 320,521	$ 99,681	$ 491,671	$ 190,341
Denominator:
Basic shares outstanding (shares)	285,276	312,315	285,976	315,021
Effect of dilutive securities (shares)	1,378	1,565	1,088	1,946
Diluted shares outstanding (shares)	286,654	313,880	287,064	316,967
Earnings per share:
Basic earnings (usd per share)	$ 1.12	$ 0.32	$ 1.72	$ 0.60
Diluted earnings (usd per share)	$ 1.12	$ 0.32	$ 1.71	$ 0.60