Fair Value Measurement (Details 4) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Minimum
Quantitative Information About Level 3 Fair Value Measurements
Discount factor (as a percent)	0.00%	0.00%
Maximum
Quantitative Information About Level 3 Fair Value Measurements
Discount factor (as a percent)	3.72%	3.28%
Asset-backed securities | Whole business securitization
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	63,000,000	63,000,000
Asset-backed securities | Whole business securitization | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Annual gross revenue projections	54,000,000	54,000,000
Liquidity discount (as a percent)	5.00%	5.00%
Asset-backed securities | Whole business securitization | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Annual gross revenue projections	96,000,000	96,000,000
Liquidity discount (as a percent)	20.00%	20.00%
Asset-backed securities | Whole business securitization | Discounted cash flow | Weighted Average
Quantitative Information About Level 3 Fair Value Measurements
Value of primary financial guaranty policy (as a percent)	43.80%	43.80%
Asset-backed securities | Investor Owned Utilities [Member]
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	162,000,000	186,000,000
Asset-backed securities | Investor Owned Utilities [Member] | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Collateral recovery period (in years)	9 months
Liquidation value	217,000,000	212,000,000
Years to liquidation	0 years	0 years
Asset-backed securities | Investor Owned Utilities [Member] | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Collateral recovery period (in years)	6 years
Liquidation value	242,000,000	242,000,000
Years to liquidation	3 years	3 years
Asset-backed securities | Investor Owned Utilities [Member] | Discounted cash flow | Weighted Average
Quantitative Information About Level 3 Fair Value Measurements
Discount factor (as a percent)	15.30%	15.30%
Asset-backed securities | XXX life insurance transactions
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	61,000,000	57,000,000
Asset-backed securities | XXX life insurance transactions | Discounted cash flow | Weighted Average
Quantitative Information About Level 3 Fair Value Measurements
Yield (as a percent)	12.00%	12.50%
Fixed maturity securities | RMBS
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	221,000,000	219,000,000
Fixed maturity securities | RMBS | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
CPR (as a percent)	0.80%	0.80%
CDR (as a percent)	4.40%	4.40%
Loss severity rate (as a percent)	47.90%	48.10%
Yield (as a percent)	3.00%	3.50%
Fixed maturity securities | RMBS | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
CPR (as a percent)	7.50%	7.50%
CDR (as a percent)	25.80%	28.60%
Loss severity rate (as a percent)	102.80%	102.80%
Yield (as a percent)	9.60%	12.80%
Fixed maturity securities | Obligations of state and political subdivisions
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	35,000,000	35,000,000
Fixed maturity securities | Obligations of state and political subdivisions | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Rate of inflation	1.00%	1.00%
Cash flow receipts (as a percent)	4.90%	4.90%
Discount factor (as a percent)	4.30%	4.30%
Collateral recovery period (in years)	1 month	1 month
Fixed maturity securities | Obligations of state and political subdivisions | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Rate of inflation	3.00%	3.00%
Cash flow receipts (as a percent)	85.80%	85.80%
Discount factor (as a percent)	9.00%	9.00%
Collateral recovery period (in years)	43 years	43 years
Other invested assets
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	7,000,000	8,000,000
Other invested assets | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Loss severity rate (as a percent)	40.00%	40.00%
Liquidity discount (as a percent)	10.00%	10.00%
Recovery on delinquent loans (as a percent)	20.00%	20.00%
Default rates (as a percent)	1.00%	1.00%
Prepayment speeds	6.00%	6.00%
Other invested assets | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Loss severity rate (as a percent)	90.00%	90.00%
Liquidity discount (as a percent)	20.00%	20.00%
Recovery on delinquent loans (as a percent)	60.00%	60.00%
Default rates (as a percent)	12.00%	12.00%
Prepayment speeds	15.00%	15.00%
Other assets
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	26,000,000	36,000,000
Other assets | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Quotes from third party pricing	51	38
Other assets | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Quotes from third party pricing	55	51
Other assets | Discounted cash flow | Weighted Average
Quantitative Information About Level 3 Fair Value Measurements
Term (in years)	3 years	3 years
Financial guaranty variable interest entities | Liabilities
Quantitative Information About Level 3 Fair Value Measurements
Fair value of liabilities as of the balance sheet date	(3,178,000,000)	(3,141,000,000)
Financial guaranty variable interest entities | Liabilities | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
CPR (as a percent)	0.40%	0.50%
CDR (as a percent)	1.80%	3.00%
Loss severity rate (as a percent)	37.50%	37.50%
Yield (as a percent)	3.00%	4.50%
Financial guaranty variable interest entities | Liabilities | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
CPR (as a percent)	11.00%	10.90%
CDR (as a percent)	28.60%	28.60%
Loss severity rate (as a percent)	106.40%	103.80%
Yield (as a percent)	20.00%	20.00%
Financial guaranty variable interest entities | Assets
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	2,813,000,000	2,688,000,000
Financial guaranty variable interest entities | Assets | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
CPR (as a percent)	0.40%	0.50%
CDR (as a percent)	1.80%	3.00%
Loss severity rate (as a percent)	37.50%	37.50%
Yield (as a percent)	3.00%	4.50%
Financial guaranty variable interest entities | Assets | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
CPR (as a percent)	11.00%	10.90%
CDR (as a percent)	28.60%	28.60%
Loss severity rate (as a percent)	106.40%	103.80%
Yield (as a percent)	20.00%	20.00%
Credit derivatives | Derivative liabilities, net
Quantitative Information About Level 3 Fair Value Measurements
Fair value of liabilities as of the balance sheet date	(2,393,000,000)	(1,793,000,000)
Credit derivatives | Derivative liabilities, net | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Year 1 loss estimates (as a percent)	0.00%	0.00%
Hedge cost (as a percent)	0.148%	0.642%
Bank profit (as a percent)	0.01%	0.01%
Internal floor (as a percent)	0.07%	0.07%
Credit derivatives | Derivative liabilities, net | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Year 1 loss estimates (as a percent)	68.00%	58.70%
Hedge cost (as a percent)	3.975%	6.782%
Bank profit (as a percent)	13.365%	13.129%
Internal floor (as a percent)	1.00%	0.60%