Fair Value Measurements (Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivative assets	$ 861	$ 804	$ 380	$ 484
Interest Rates
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivative assets	8	13	15
Derivative liabilities	$ 5	5	3
Equity Contract
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivative assets		0	262
Derivative liabilities		$ 91	$ 61