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	Mar. 31, 2018	Dec. 31, 2017
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	$ 981	$ 803
Securities loaned	400	111
Gross amount of recognized liabilities for repurchase agreements and securities loaned	1,381	914
Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	717	773
Securities loaned	400	111
Gross amount of recognized liabilities for repurchase agreements and securities loaned	1,117	884
Less Than 30 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	264	30
Gross amount of recognized liabilities for repurchase agreements and securities loaned	264	30
Corporate Debt Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	366	104
Securities loaned	400	111
Corporate Debt Securities [Member] | Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	279	104
Securities loaned	400	111
Corporate Debt Securities [Member] | Less Than 30 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	87
U.S. Treasury and Agencies [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	58	25
U.S. Treasury and Agencies [Member] | Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	58	25
Residential Mortgage-Backed Securities [Member] | Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	557	674
Residential Mortgage-Backed Securities [Member] | Overnight and Continuous [Member] | Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	380	644
Residential Mortgage-Backed Securities [Member] | Less Than 30 Days [Member] | Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	$ 177	$ 30