Fair Value Of Financial Instruments (Quantitative Information Regarding Significant Unobservable Inputs For Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value			$ 3,937	$ 4,188
Commercial Mortgage-Backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value			25	24
Variable Interest Entity [Member] | Commercial Mortgage-Backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value			15	22
Credit Derivatives [Member] | Commercial Mortgage-Backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Techniques	BET Model	BET Model
Fair Value	1,851	1,590
Credit Derivatives [Member] | Multi-Sector CDO [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Techniques	Direct Price Model	Direct Price Model
Fair Value	594	525
Credit Derivatives [Member] | Other [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Techniques	BET Model	BET Model
Fair Value	549	806
Loans Receivable [Member] | Variable Interest Entity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Techniques	Quoted market prices adjusted for financial guarantees provided to VIE obligations	Quoted market prices adjusted for financial guarantees provided to VIE obligations
Fair Value	1,819	1,881	2,025	2,046
Loans Receivable [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee Asset [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	0.00%	0.00%
Loans Receivable [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee Asset [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	21.00%	14.00%
Loans Receivable [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee Asset [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	4.00%	3.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Techniques	Discounted cash flow	Discounted cash flow
Fair Value	1,176	1,086	1,076	1,077
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Recovery Rates [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	10.00%	10.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Recovery Rates [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	75.00%	75.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Recovery Rates [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	46.00%	47.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Breach Rates [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	67.00%	66.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Breach Rates [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	94.00%	94.00%
Loan Repurchase Commitments [Member] | Variable Interest Entity [Member] | Breach Rates [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	78.00%	78.00%
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Techniques	Quoted market prices of VIE assets adjusted for financial guarantees provided	Quoted market prices of VIE assets adjusted for financial guarantees provided
Fair Value	$ 1,901	$ 1,932
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee Liability [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	0.00%	0.00%
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee Liability [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	34.00%	23.00%
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] | Impact Of Financial Guarantee Liability [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	8.00%	6.00%
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | Recovery Rates [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	21.00%	21.00%
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | Recovery Rates [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	90.00%	90.00%
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | Recovery Rates [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	51.00%	51.00%
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | Nonperformance Risk [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	20.00%	19.00%
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | Nonperformance Risk [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	48.00%	59.00%
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | Nonperformance Risk [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	48.00%	58.00%
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | Weighted Average Life [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range (in years)	0.1	0.1
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | Weighted Average Life [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range (in years)	33.5	5.6
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | Weighted Average Life [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range (in years)	4.3	4.4
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | CMBS Spreads [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	0.60%	1.00%
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | CMBS Spreads [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	23.50%	23.00%
Credit Derivative Liabilities [Member] | Commercial Mortgage-Backed [Member] | CMBS Spreads [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	13.10%	13.00%
Credit Derivative Liabilities [Member] | Multi-Sector CDO [Member] | Nonperformance Risk [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	48.00%	59.00%
Credit Derivative Liabilities [Member] | Multi-Sector CDO [Member] | Nonperformance Risk [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	48.00%	59.00%
Credit Derivative Liabilities [Member] | Multi-Sector CDO [Member] | Nonperformance Risk [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	48.00%	59.00%
Credit Derivative Liabilities [Member] | Other [Member] | Recovery Rates [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	42.00%	42.00%
Credit Derivative Liabilities [Member] | Other [Member] | Recovery Rates [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	70.00%	75.00%
Credit Derivative Liabilities [Member] | Other [Member] | Recovery Rates [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	47.00%	47.00%
Credit Derivative Liabilities [Member] | Other [Member] | Nonperformance Risk [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	37.00%	42.00%
Credit Derivative Liabilities [Member] | Other [Member] | Nonperformance Risk [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	48.00%	59.00%
Credit Derivative Liabilities [Member] | Other [Member] | Nonperformance Risk [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range Percentage	48.00%	58.00%
Credit Derivative Liabilities [Member] | Other [Member] | Weighted Average Life [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range (in years)	0.4	0.1
Credit Derivative Liabilities [Member] | Other [Member] | Weighted Average Life [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range (in years)	19.6	19.6
Credit Derivative Liabilities [Member] | Other [Member] | Weighted Average Life [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Range (in years)	2.8	3.0