Netting Arrangements for Certain Financial Instruments and Securities Financing Activities - Summary of Maturities by Category of Collateral Pledged for Repurchase Agreements and Securities Loaned Transactions (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	$ 758	$ 801
Securities loaned	359	223
Gross amount of recognized liabilities for repurchase agreements and securities loaned	1,117	1,024
Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	713	771
Securities loaned	359	223
Gross amount of recognized liabilities for repurchase agreements and securities loaned	1,072	994
Less Than 30 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	45	30
Gross amount of recognized liabilities for repurchase agreements and securities loaned	45	30
Corporate Debt Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements		30
Securities loaned	359	223
Corporate Debt Securities [Member] | Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements		30
Securities loaned	359	223
U.S. Treasury and Agencies [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	91	60
U.S. Treasury and Agencies [Member] | Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	91	60
Mortgage-Backed Securities Residential [Member] | Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	667	711
Mortgage-Backed Securities Residential [Member] | Overnight and Continuous [Member] | Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	622	681
Mortgage-Backed Securities Residential [Member] | Less Than 30 Days [Member] | Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	$ 45	$ 30