Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Feb. 01, 2017	Dec. 31, 2014
Numerator:
Income from continuing operations	$ 158,857	$ 126,720	$ 409,949	$ 305,874
Loss from discontinued operations, net of income taxes	0	0	0	(42,704)
Net income	$ 158,857	$ 126,720	$ 409,949	$ 263,170
Denominator:
Denominator for basic net earnings per share - weighted-average shares outstanding (shares)	135,055	151,156	136,752	151,896
Effect of dilutive employee stock awards (shares)	2,731	1,987	2,594	2,538
Effect of dilutive Convertible Notes (shares)	6,999	1,484	5,892	1,950
Effect of dilutive warrants (shares)	2,783	0	1,316	0
Denominator for net diluted earnings per share - weighted-average shares outstanding (shares)	147,568	154,627	146,554	156,384
Income from continuing operations, basic (in dollars per share)	$ 1.18	$ 0.84	$ 3.00	$ 2.01
Loss from discontinued operations, basic (in dollars per share)	0.00	0.00	0.00	(0.28)
Basic earnings per share (in dollars per share)	1.18	0.84	3.00	1.73
Income from continuing operations, diluted (in dollars per share)	1.08	0.82	2.80	1.96
Loss from discontinued operations, diluted (in dollars per share)	0.00	0.00	0.00	(0.28)
Diluted earnings per share (in dollars per share)	1.08	$ 0.82	2.80	$ 1.68
GetGo, Inc. [Member]
Denominator:
Convertible debt, conversion price (in dollars per share)					$ 71.91
Senior Notes Due 2019 [Member]
Denominator:
Strike price of warrants (in dollars per share)	$ 95.25		$ 95.25
Stated interest rate percentage						0.50%
Senior Notes Due 2019 [Member] | GetGo, Inc. [Member]
Denominator:
Convertible debt, conversion price (in dollars per share)					$ 71.91