Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investments	$ 268	$ 261	$ 256	$ 175	$ 179
Investments in equity affiliates
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investments	135				46
Marketable equity securities - Trading (See Note 9) | Trading Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investments	79				78
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investments	$ 54				$ 55