Schedule of Financial Instruments Owned and Pledged as Collateral (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of Repurchase Agreements Secured by Trading Assets		$ 1,238	$ 975
Trading Assets [Member] | Repurchase Agreements [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Security Owned and Pledged as Collateral, Fair Value	[1]	1,284	1,016
Trading Assets [Member] | Derivative [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Security Owned and Pledged as Collateral, Fair Value		76	72
Trading Assets [Member] | Equity Trading [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Security Owned and Pledged as Collateral, Fair Value		$ 40	$ 41

[1] Repurchase agreements secured by collateral totaled $1.2 billion and $975 million at September 30, 2018 and December 31, 2017, respectively.