Basis of Presentation (Earnings per share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income	$ 241,041	$ 166,757	$ 324,089	$ 170,751	$ 407,798	$ 494,841
Less: earnings distributed to participating securities	(305)		(300)		(613)	(595)
Less: undistributed earnings allocated to participating securities	(2,089)		(3,284)		(3,588)	(2,584)
Numerator for basic earnings per share	238,647		320,505		403,597	491,662
Add back: undistributed earnings allocated to participating securities	2,089		3,284		3,588	2,584
Less: undistributed earnings reallocated to participating securities	(2,082)		(3,268)		(3,576)	(2,575)
Numerator for diluted earnings per share	$ 238,654		$ 320,521		$ 403,609	$ 491,671
Denominator:
Basic shares outstanding (shares)	276,652		285,276		277,142	285,976
Effect of dilutive securities (shares)	932		1,378		967	1,088
Diluted shares outstanding (shares)	277,584		286,654		278,109	287,064
Earnings per share:
Basic earnings (usd per share)	$ 0.86		$ 1.12		$ 1.46	$ 1.72
Diluted earnings (usd per share)	$ 0.86		$ 1.12		$ 1.45	$ 1.71