Derivative Instruments - Summary of Cumulative Hedging Adjustments and the Carrying Amount of Assets (Liabilities) Designated in Fair Value Hedges (Detail) - Fair Value Hedges [Member] - Long-term Debt Securities [Member] - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Carrying amount of the hedged assets (liabilities), Long-term debt		$ 4,584
Cumulative hedging Adjustment included in the carrying amount of the hedged assets (liabilities)	$ (36)	$ (8)