Netting Arrangements for Certain Financial Instruments - Information on Company's Accounting Netting Adjustments and Items Not Offset in Consolidated Balance Sheet But Available for Offset in Event of Default (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financial Instruments [Line Items]
Gross recognized assets	$ 2,268	$ 2,277
Gross amounts assets offset in consolidated balance sheet	(515)	(418)
Net amounts assets presented in consolidated balance sheet	1,753	1,859
Gross financial instrument asset amounts not offset in consolidated balance sheet	(58)	(75)
Gross collateral received amounts not offset in consolidated balance sheet	(622)	(675)
Net amount assets	1,073	1,109
Gross recognized liabilities	4,523	5,567
Gross amounts liabilities offset in consolidated balance sheet	(1,472)	(1,549)
Net amounts liabilities presented in consolidated balance sheet	3,051	4,018
Gross financial instrument liability amounts not offset in consolidated balance sheet	(58)	(75)
Gross collateral pledged amounts not offset in consolidated balance sheet	(2,361)	(3,345)
Net amount liabilities	632	598
Derivative Assets [Member]
Financial Instruments [Line Items]
Gross recognized assets	1,596	1,546
Gross amounts assets offset in consolidated balance sheet	(515)	(418)
Net amounts assets presented in consolidated balance sheet	1,081	1,128
Gross financial instrument asset amounts not offset in consolidated balance sheet	(27)	(31)
Net amount assets	1,054	1,097
Reverse Repurchase Agreements [Member]
Financial Instruments [Line Items]
Gross recognized assets	44	363
Net amounts assets presented in consolidated balance sheet	44	363
Gross financial instrument asset amounts not offset in consolidated balance sheet	(31)	(44)
Gross collateral received amounts not offset in consolidated balance sheet	(13)	(319)
Securities Borrowed [Member]
Financial Instruments [Line Items]
Gross recognized assets	628	368
Net amounts assets presented in consolidated balance sheet	628	368
Gross collateral received amounts not offset in consolidated balance sheet	(609)	(356)
Net amount assets	19	12
Derivative Liabilities [Member]
Financial Instruments [Line Items]
Gross recognized liabilities	2,131	2,178
Gross amounts liabilities offset in consolidated balance sheet	(1,472)	(1,549)
Net amounts liabilities presented in consolidated balance sheet	659	629
Gross financial instrument liability amounts not offset in consolidated balance sheet	(27)	(31)
Net amount liabilities	632	598
Repurchase Agreements [Member]
Financial Instruments [Line Items]
Gross recognized liabilities	2,392	3,389
Net amounts liabilities presented in consolidated balance sheet	2,392	3,389
Gross financial instrument liability amounts not offset in consolidated balance sheet	(31)	(44)
Gross collateral pledged amounts not offset in consolidated balance sheet	$ (2,361)	$ (3,345)