Summary of Significant Accounting Policies - Investments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of Significant Accounting Policies
Equity method investments	$ 127	$ 173
Cost method investments	142	147
Investments in unconsolidated entities	269	320
Total interest costs	383	394	$ 407
Total capitalized interest costs	$ 15	$ 9	$ 16