Other Financial Information - Schedule of Other Noncurrent Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Other Financial Information [Abstract]
Gas supply prepayment	$ 458	$ 510
Prepaid gas transmission contract	275	300
Value-added tax credits	210	340
Goodwill	151	154
Deferred mining costs, net	123	139
Prepaid pension benefit	63	72
Intangibles, net	57	62
Other	138	142
Other assets, noncurrent, total	$ 1,475	$ 1,719