Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Numerator:
Net income	$ 143	$ 185	$ 228	$ 185	$ 276	$ 178	$ 556	$ 639
Denominator:
Shares outstanding at the beginning of the period (in shares)		57.8			58.7				58.0	61.9
Weighted-average number of shares issued:
Stock purchases (in shares)	0.0			0.0			(0.5)	(2.8)
Stock-based compensation (in shares)	0.0			0.0			0.3	0.1
Denominator for basic net income per share (in shares)	57.8			58.7			57.8	59.2
Effect of dilutive securities:
Stock-based compensation (in shares)	0.7			0.9			0.7	0.8
Denominator for diluted net income per share (in shares)	58.5			59.6			58.5	60.0
Net income per share:
Net income per share - Basic (in dollars per share)	$ 2.49			$ 3.14			$ 9.63	$ 10.80
Net income per share - Diluted (in dollars per share)	$ 2.46			$ 3.10			$ 9.51	$ 10.65