Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ 673	$ 737	$ 707
Denominator:
Shares outstanding at the beginning of the period (in shares)		61.9	62.1	59.3
Weighted-average number of shares issued:
Stock purchases (in shares)	(3.0)	0.0	0.0
Stock-based compensation (in shares)	0.1	0.1	0.2
Exchange of convertible senior notes (in shares)	0.0	0.0	1.4
Conversion of convertible senior notes (in shares)	0.0	0.0	0.2
Denominator for basic net income per share (in shares)	59.0	62.2	61.1
Effect of dilutive securities:
Share-based compensation (in shares)	0.9	0.6	0.3
Warrants (in shares)	0.0	1.4	4.8
Convertible senior notes (in shares)	0.0	0.0	0.4
Denominator for diluted net income per share (in shares)	59.9	64.2	66.6
Net income per share:
Basic (in dollars per share)	$ 11.40	$ 11.85	$ 11.57
Diluted (in dollars per share)	$ 11.23	$ 11.47	$ 10.61
Convertible senior notes
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Stated interest rate	1.625%