Other Financial Information - Schedule of Other Noncurrent Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other Financial Information [Abstract]
Gas supply prepayment	$ 510	$ 471
Value-added tax credits	340	287
Prepaid gas transmission contract	300	270
Goodwill	154	155
Deferred mining costs, net	139	127
Prepaid pension benefit	72	43
Intangibles, net	62	135
Other	142	180
Other assets, noncurrent, total	$ 1,719	$ 1,668