Acquisitions - Results of Operations from Properties Included in Consolidated Statement of Comprehensive Income (Detail) - USD ($) $ 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
Business Acquisition [Line Items]
Total revenues	$ 686	$ 688	$ 733	$ 684	$ 670	$ 671	$ 725	$ 661	$ 2,791	$ 2,727	$ 2,688
Income before income taxes									$ 285	$ 221	417
Sellers Affiliated with Blackstone and an Unrelated Third Party [Member]
Business Acquisition [Line Items]
Total revenues											316
Income before income taxes											$ 58