Equity-Based Compensation - PSUs (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Performance Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		3 years	3 years
Vested shares issued, percentage of the established target			200.00%
Range of potential payout (as a percent)		100.00%
Unvested, Units, beginning of year		1,505
Granted, Units		400
Vested, Units	(1,140,000)	(570)
Forfeited, Units		(36)
Unvested, Units, end of year		1,299	1,505
Unvested, Weighted Average Per Share Fair Value, beginning of year		$ 68.98
Granted, Weighted Average Per Share Fair Value		84.54
Vested, Weighted Average Per Share Fair Value		55.22
Forfeited, Weighted Average Per Share Fair Value		85.57
Unvested, Weighted Average Per Share Fair Value, end of year		$ 84.78	$ 68.98
Shares issued under the plan	721,000
Total fair market value of vested awards		$ 80	$ 50	$ 35
Deferred Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested deferred units outstanding		203,000
Minimum [Member] | Performance Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of potential payout (as a percent)		0.00%
Maximum [Member] | Performance Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of potential payout (as a percent)		200.00%