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, 2019	Dec. 31, 2018
Offsetting [Abstract]
Cash collateral netted against derivative assets	$ 760	$ 236
Derivative assets not subject to netting arrangements	$ 62	$ 23