Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Income (Numerator)
Basic earnings per share - net income (in dollars)	$ 153,258	$ 157,397	$ 508,008	$ 362,103
Diluted earnings per share - net income (in dollars)	$ 153,258	$ 157,397	$ 508,008	$ 362,103
Weighted average common shares outstanding (in shares)
Weighted average common shares outstanding - basic (in shares)	239,066	243,761	241,117	243,539
Dilutive common share equivalents (in shares)	1,814	1,921	1,699	1,688
Weighted average common shares outstanding - diluted (in shares)	240,880	245,682	242,816	245,227
Per Share Amount
Basic (in dollars per share)	$ 0.64	$ 0.65	$ 2.11	$ 1.49
Diluted (in dollars per share)	$ 0.64	$ 0.64	$ 2.09	$ 1.48