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 Liabilities But Available for Offset in Event of Default (Parenthetical) (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Offsetting [Abstract]
Cash collateral netted against derivative liabilities	$ 344	$ 528
Derivative liabilities not subject to netting arrangements	$ 122	$ 738