Note Debt Securities Available for Sale (Amortized cost, gross unrealized gains and losses, approximate fair value, weighted average yield and contractual maturities of debt securities AFS) (Parenthetical) (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale debt securities
Schedule of Available-for-sale Securities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Securities	$ 8,200	$ 6,600
Securities Investment
Schedule of Available-for-sale Securities [Line Items]
Pledged Financial Instruments, Not Separately Reported, Securities for Other Debt Facilities	$ 7,500	$ 5,600