Net Income per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ 196	$ 198	$ 202	$ 107	$ 394	$ 309
Denominator:
Shares outstanding at the beginning of the period (in shares)		62.1		61.2			62.1	59.3
Exchange of 1.625% Notes (in shares)	0.0		0.0		0.0	1.1
Stock-based compensation (in shares)	0.0		0.0		0.0	0.1
Denominator for net income per share, basic (in shares)	62.1		61.2		62.1	60.5
Effect of dilutive securities:
1.125% Warrants (in shares)	1.3		4.9		2.4	4.8
1.625% Notes (in shares)	0.0		0.4		0.0	0.5
Stock-based compensation (in shares)	0.6		0.2		0.6	0.2
Denominator for net income per share, diluted (in shares)	64.0		66.7		65.1	66.0
Net income per share:
Basic (in dollars per share)	$ 3.15		$ 3.29		$ 6.34	$ 5.10
Diluted (in dollars per share)	$ 3.06		$ 3.02		$ 6.04	$ 4.68
Potentially dilutive common shares excluded from calculations: (1)
Stock-based compensation (in shares)	0.0		0.4		0.0	0.4
1.125% Warrants
Potentially dilutive common shares excluded from calculations: (1)
Stated percentage of warrants	1.125%	1.125%	1.125%		1.125%	1.125%
1.625% Notes
Potentially dilutive common shares excluded from calculations: (1)
Stated percentage of warrants	1.625%				1.625%