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, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	$ 965	$ 948
Securities loaned	199	47
Gross amount of recognized liabilities for repurchase agreements and securities loaned	1,164
Overnight and Continuous [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	694
Securities loaned	199
Gross amount of recognized liabilities for repurchase agreements and securities loaned	893
Less Than 30 Days [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	271
Gross amount of recognized liabilities for repurchase agreements and securities loaned	271
U.S. Treasury and Agencies [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	52
U.S. Treasury and Agencies [Member] | Overnight and Continuous [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	40
U.S. Treasury and Agencies [Member] | Less Than 30 Days [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	12
Corporate Debt Securities [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Securities loaned	199
Corporate Debt Securities [Member] | Overnight and Continuous [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Securities loaned	199
Mortgage-Backed Securities Residential [Member] | Agency [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	913
Mortgage-Backed Securities Residential [Member] | Overnight and Continuous [Member] | Agency [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	654
Mortgage-Backed Securities Residential [Member] | Less Than 30 Days [Member] | Agency [Member]
Offsetting Derivatives Financial Assets And Financial Liabilities [Line Items]
Repurchase agreements	$ 259