Selected Quarterly Financial Information (unaudited) (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 810		$ 672		$ 703		$ 659		$ 686		$ 652		$ 731		$ 668		$ 2,844		$ 2,737		$ 2,791
Operating income	148		38		111		129		97		84		149		174		426		504		371
Net income	126		9		84		97		55		55		218		149		316		477		2,631
Net income attributable to stockholders	$ 123		$ 5		$ 82		$ 96		$ 54		$ 52		$ 216		$ 150		$ 306	[1]	$ 472	[1]	$ 2,625	[1]
Earnings per share - Basic	$ 0.51	[2]	$ 0.02	[2]	$ 0.40	[2]	$ 0.48	[2]	$ 0.27	[2]	$ 0.26	[2]	$ 1.07	[2]	$ 0.71	[2]	$ 1.44	[2],[3]	$ 2.32	[2],[3]	$ 12.38	[3]
Earnings per share - Diluted	$ 0.51	[2]	$ 0.02	[2]	$ 0.40	[2]	$ 0.48	[2]	$ 0.27	[2]	$ 0.26	[2]	$ 1.07	[2]	$ 0.71	[2]	$ 1.44	[2],[3]	$ 2.31	[2],[3]	$ 12.21	[3]

[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] 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 necessarily equal the EPS for the full year.

[3] Per share amounts are calculated based on unrounded numbers.