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	Jun. 30, 2015	Dec. 31, 2014
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	$ 975	$ 948
Securities loaned	190	$ 47
Gross amount of recognized liabilities for repurchase agreements and securities loaned	1,165
Overnight and Continuous [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	523
Securities loaned	190
Gross amount of recognized liabilities for repurchase agreements and securities loaned	713
Less Than 30 Days [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	452
Securities loaned	0
Gross amount of recognized liabilities for repurchase agreements and securities loaned	452
U.S. Treasury and Agencies [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	40
U.S. Treasury and Agencies [Member] | Overnight and Continuous [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	26
U.S. Treasury and Agencies [Member] | Less Than 30 Days [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	14
Corporate Debt Securities [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Securities loaned	190
Corporate Debt Securities [Member] | Overnight and Continuous [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Securities loaned	190
Corporate Debt Securities [Member] | Less Than 30 Days [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Securities loaned	0
Mortgage-Backed Securities Residential [Member] | Agency [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	935
Mortgage-Backed Securities Residential [Member] | Overnight and Continuous [Member] | Agency [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	497
Mortgage-Backed Securities Residential [Member] | Less Than 30 Days [Member] | Agency [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	$ 438