Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Notional or contractual amount	$ 61,950	$ 57,812
Fair value of derivatives assets	988	1,848
Fair value of derivatives liabilities	587	400
Gain (net after-tax) recorded in AOCI related to derivatives designated as cash flow hedges expected to be reclassified to earnings over the next 12 months	164.0
Maximum length of time over which forecasted transactions were hedged, years	5 years
Fair value, derivative liabilities	587	400
Contingent collateral required upon credit rating falling below investment grade	1	4
Independent margin	67	67
Cash collateral from derivatives counterparties	288	922
Securities received from derivatives counterparties, ability to repledge	37	238
Cumulative counterparty credit risk valuation adjustment	9	9
Cumulative credit risk valuation adjustment related to our credit quality	4	1
Credit-Risk-Related Contingency Features [Member]
Derivative [Line Items]
Fair value, derivative liabilities	1	7
Collateral posted for derivative instruments	$ 144	$ 109
Cash Flow Hedges [Member]
Derivative [Line Items]
Higher remaining maturity range	2018