Netting Arrangements for Certain Financial Instruments and Securities Financing Activities - Information on Company's Accounting Netting Adjustments and Items Not Offset in Consolidated Balance Sheet Assets But Available for Offset in Event of Default (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Offsetting [Abstract]
Cash collateral netted against derivative assets	$ 229	$ 50
Derivative assets not subject to netting arrangements	$ 23	$ 723