Selected Quarterly Financial Information (unaudited) (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 686		$ 688		$ 733		$ 684		$ 670		$ 671		$ 725		$ 661		$ 2,791		$ 2,727		$ 2,688
Operating income	65		89		123		94		87		110		142		80		371		419		586
Net income	61		105		115		2,350		17		37		62		23		2,631		139		299
Net income attributable to stockholders	$ 60		$ 103		$ 112		$ 2,350		$ 17		$ 34		$ 60		$ 22		$ 2,625	[1]	$ 133	[1],[2]	$ 292	[1],[2]
Earnings per share - Basic	$ 0.28	[3]	$ 0.48	[3]	$ 0.52	[3]	$ 11.65	[3]	$ 0.09	[4]	$ 0.17	[4]	$ 0.30	[4]	$ 0.11	[4]	$ 12.38	[3],[5]	$ 0.67	[2],[4],[5]	$ 1.48	[2],[5]
Earnings per share - Diluted	$ 0.28	[3]	$ 0.48	[3]	$ 0.52	[3]	$ 11.02	[3]	$ 0.09	[4]	$ 0.17	[4]	$ 0.30	[4]	$ 0.11	[4]	$ 12.21	[3],[5]	$ 0.67	[2],[4],[5]	$ 1.48	[2],[5]

[1] Includes the derecognition and remeasurement of deferred tax assets and liabilities for the year ended December 31, 2017 of $2,347 million associated with our intent to be taxed as a REIT.

[2] For 2016 and 2015, basic and diluted earnings per share were calculated using the number of shares of common stock outstanding upon the completion of the spin-off.

[3] Per share amounts are calculated based on unrounded numbers and are calculated independently for each period presented, therefore, the sum of the quarterly EPS does not equal the EPS for the full year.

[4] Basic and diluted earnings per share were calculated using the number of shares of common stock outstanding upon the completion of the spin-off. Per share amounts are calculated based on unrounded numbers and are calculated independently for each period presented.

[5] Per share amounts are calculated based on unrounded numbers and are calculated independently for each period presented.