Fair Value Measurement (Details 4) (USD $)	6 Months Ended
	Jun. 30, 2012
Asset-backed securities | Whole business securitization
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	$ 46,700,000
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
Liquidity discount (as a percent)	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
Liquidity discount (as a percent)	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%
Asset-backed securities | Investor-owned utility
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	191,400,000
Asset-backed securities | Investor-owned utility | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Liquidation value	174,000,000
Discount factor (as a percent)	0.00%
Asset-backed securities | Investor-owned utility | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Liquidation value	281,000,000
Years to liquidation	3 years
Discount factor (as a percent)	11.00%
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	35,600,000
Asset-backed securities | XXX life insurance transactions | Discounted cash flow | Weighted Average
Quantitative Information About Level 3 Fair Value Measurements
Discount factor (as a percent)	50.00%
Yield (as a percent)	15.00%
Asset-backed securities | Other asset-backed
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	300,000
Asset-backed securities | Other asset-backed | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Loss severity rate (as a percent)	40.00%
CPR (as a percent)	0.00%
CDR (as a percent)	2.00%
Yield (as a percent)	3.00%
Asset-backed securities | Other asset-backed | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Loss severity rate (as a percent)	100.00%
CPR (as a percent)	10.00%
CDR (as a percent)	10.00%
Yield (as a percent)	15.00%
Fixed maturity securities | RMBS
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	166,700,000
Fixed maturity securities | RMBS | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Loss severity rate (as a percent)	48.90%
CPR (as a percent)	0.00%
CDR (as a percent)	3.70%
Yield (as a percent)	6.70%
Fixed maturity securities | RMBS | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Loss severity rate (as a percent)	104.40%
CPR (as a percent)	7.40%
CDR (as a percent)	41.10%
Yield (as a percent)	16.00%
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	9,700,000
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%
Timing of collateral sales	3 years
Assumed sale proceeds (as a percent)	0.00%
Collateral recovery period (in years)	3 years
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%
Timing of collateral sales	11 years
Assumed sale proceeds (as a percent)	11.90%
Collateral recovery period (in years)	11 years
Other invested assets
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	9,800,000
Other invested assets | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Prepayment speeds	6.00%
Liquidity discount (as a percent)	10.00%
Recovery on delinquent loans (as a percent)	20.00%
Default rates (as a percent)	1.00%
Loss severity rate (as a percent)	40.00%
Other invested assets | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Prepayment speeds	15.00%
Liquidity discount (as a percent)	20.00%
Recovery on delinquent loans (as a percent)	60.00%
Default rates (as a percent)	10.00%
Loss severity rate (as a percent)	90.00%
Other assets
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	44,200,000
Other assets | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Quotes from third party pricing	38
Other assets | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Quotes from third party pricing	42
Other assets | Discounted cash flow | Weighted Average
Quantitative Information About Level 3 Fair Value Measurements
Term	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	(3,281,300,000)
Financial guaranty variable interest entities | Liabilities | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Loss severity rate (as a percent)	25.50%
CPR (as a percent)	0.00%
CDR (as a percent)	1.80%
Yield (as a percent)	4.70%
Financial guaranty variable interest entities | Liabilities | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Loss severity rate (as a percent)	110.10%
CPR (as a percent)	13.20%
CDR (as a percent)	39.10%
Yield (as a percent)	22.40%
Financial guaranty variable interest entities | Assets
Quantitative Information About Level 3 Fair Value Measurements
Fair value of assets as of the balance sheet date	2,726,000,000
Financial guaranty variable interest entities | Assets | Discounted cash flow | Minimum
Quantitative Information About Level 3 Fair Value Measurements
Loss severity rate (as a percent)	25.50%
CPR (as a percent)	0.00%
CDR (as a percent)	1.80%
Yield (as a percent)	4.70%
Financial guaranty variable interest entities | Assets | Discounted cash flow | Maximum
Quantitative Information About Level 3 Fair Value Measurements
Loss severity rate (as a percent)	110.10%
CPR (as a percent)	13.20%
CDR (as a percent)	39.10%
Yield (as a percent)	22.40%
Credit derivatives | Derivative liabilities, net
Quantitative Information About Level 3 Fair Value Measurements
Fair value of liabilities as of the balance sheet date	$ (1,666,000,000)
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%
Hedge cost (in bps)	1.04%
Bank profit (in bps)	0.01%
Internal floor (in bps)	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)	69.00%
Hedge cost (in bps)	9.04%
Bank profit (in bps)	12.87%
Internal floor (in bps)	0.40%