Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	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	Dec. 31, 2016
Numerator:
Net income (loss)	$ 168	$ 175	$ 196	$ 198	$ 201	$ 197	$ 202	$ 107	$ 737	$ 707	$ (512)
Denominator:
Shares outstanding at the beginning of the period (in shares)					62.1					62.1	59.3	55.8
Weighted-average number of shares issued:
Exchange of convertible senior notes (in shares)									0.0	1.4	0.1
Conversion of convertible senior notes (in shares)									0.0	0.2	0.0
Stock-based compensation (in shares)									0.1	0.2	0.5
Denominator for basic net income (loss) per share (in shares)									62.2	61.1	56.4
Effect of dilutive securities:
Warrants (in shares)									1.4	4.8	0.0
Convertible senior notes (in shares)									0.0	0.4	0.0
Share-based compensation (in shares)									0.6	0.3	0.0
Denominator for diluted net (loss) income per share (in shares)									64.2	66.6	56.4
Net income (loss) per share:
Basic (in dollars per share)	$ 2.70	$ 2.81	$ 3.15	$ 3.19	$ 3.24	$ 3.22	$ 3.29	$ 1.79	$ 11.85	$ 11.57	$ (9.07)
Diluted (in dollars per share)	$ 2.67	$ 2.75	$ 3.06	$ 2.99	$ 3.01	$ 2.90	$ 3.02	$ 1.64	$ 11.47	$ 10.61	$ (9.07)
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Stock-based compensation (in shares)									0.0	0.0	0.3
Convertible senior notes
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Potentially dilutive common shares excluded from calculations (in shares)									0.0	0.0	0.4
Stated interest rate	1.625%								1.625%
Warrants
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Potentially dilutive common shares excluded from calculations (in shares)									0.0	0.0	1.9