Fair Value Measurement - Quantitative Information - Assets (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount factor (as a percent)	0.00%		0.00%
Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount factor (as a percent)	3.68%		4.44%
Obligations of state and political subdivisions | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 38	[1]	$ 36	[1]
Obligations of state and political subdivisions | Minimum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Rate of inflation (as a percent)	1.00%		1.00%
Cash flow receipts (as a percent)	0.50%		0.50%
Discount factor (as a percent)			4.60%
Yield (as a percent)	4.60%
Collateral recovery period (in years)	3 months		1 month
Obligations of state and political subdivisions | Maximum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Rate of inflation (as a percent)	3.00%		3.00%
Cash flow receipts (as a percent)	73.40%		60.90%
Discount factor (as a percent)			9.00%
Yield (as a percent)	8.00%
Collateral recovery period (in years)	34 years		10 years
Obligations of state and political subdivisions | Weighted Average | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Rate of inflation (as a percent)	2.00%		2.00%
Cash flow receipts (as a percent)	62.20%		51.10%
Discount factor (as a percent)			8.00%
Yield (as a percent)	7.30%
Collateral recovery period (in years)	28 years		3 years
Corporate securities | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	97	[1]	136	[1]
Yield (as a percent)	13.90%		8.30%
RMBS | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	512	[1]	290	[1]
RMBS | Minimum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	0.80%		2.50%
Conditional prepayment rate (as a percent)	0.30%		1.00%
Conditional default rate (as a percent)	2.50%		5.00%
Loss severity rate (as a percent)	30.10%		48.10%
RMBS | Maximum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	11.50%		9.40%
Conditional prepayment rate (as a percent)	31.30%		15.80%
Conditional default rate (as a percent)	13.10%		25.80%
Loss severity rate (as a percent)	100.00%		102.50%
RMBS | Weighted Average | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	7.50%		5.70%
Conditional prepayment rate (as a percent)	3.00%		4.10%
Conditional default rate (as a percent)	6.10%		17.90%
Loss severity rate (as a percent)	82.20%		87.20%
Investor Owned Utilities | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	123	[1]	141	[1]
Discount factor (as a percent)	7.00%		15.30%
Collateral recovery period (in years)	4 years
Investor Owned Utilities | Minimum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Collateral recovery period (in years)			12 months
Liquidation value			195
Years to liquidation			0 years
Investor Owned Utilities | Maximum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Collateral recovery period (in years)			6 years
Liquidation value			245
Years to liquidation			3 years
Investor Owned Utilities | Weighted Average | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Collateral recovery period (in years)			3 years 6 months
Liquidation value			228
Years to liquidation			2 years
Asset Backed Securities Other | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	143	[1]	127	[1]
Yield (as a percent)	7.50%		12.50%
Other invested assets | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	82	[1]	8	[1]
Liquidity discount (as a percent)	20.00%
Recovery on delinquent loans (as a percent)	40.00%
Other invested assets | Minimum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Conditional prepayment rate (as a percent)	5.00%		6.00%
Conditional default rate (as a percent)	0.00%		1.00%
Loss severity rate (as a percent)	40.00%		40.00%
Liquidity discount (as a percent)			10.00%
Recovery on delinquent loans (as a percent)			20.00%
Net asset value (per share)	$ 1,085
Other invested assets | Maximum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Conditional prepayment rate (as a percent)	15.00%		15.00%
Conditional default rate (as a percent)	7.00%		10.00%
Loss severity rate (as a percent)	90.00%		90.00%
Liquidity discount (as a percent)			20.00%
Recovery on delinquent loans (as a percent)			60.00%
Net asset value (per share)	$ 1,091
Other invested assets | Weighted Average | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Conditional prepayment rate (as a percent)	12.10%		13.10%
Conditional default rate (as a percent)	6.20%		3.20%
Loss severity rate (as a percent)	76.30%		73.50%
Liquidity discount (as a percent)			20.00%
Recovery on delinquent loans (as a percent)			40.00%
Net asset value (per share)	$ 1,088
Financial guaranty, variable interest entities | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	1,296	[1]	2,565	[1]
Financial guaranty, variable interest entities | Minimum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.50%		3.50%
Conditional prepayment rate (as a percent)	0.00%		0.30%
Conditional default rate (as a percent)	1.20%		3.00%
Loss severity rate (as a percent)	45.00%		37.50%
Financial guaranty, variable interest entities | Maximum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	17.60%		10.20%
Conditional prepayment rate (as a percent)	11.40%		11.80%
Conditional default rate (as a percent)	13.30%		25.80%
Loss severity rate (as a percent)	100.00%		102.00%
Financial guaranty, variable interest entities | Weighted Average | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	8.30%		5.40%
Conditional prepayment rate (as a percent)	3.60%		3.60%
Conditional default rate (as a percent)	6.50%		13.60%
Loss severity rate (as a percent)	85.40%		94.60%
Other Assets | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 35	[1]	$ 46	[1]
Other Assets | Minimum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Quotes from third party pricing (in dollars per share)	$ 52		$ 47
Other Assets | Maximum | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Quotes from third party pricing (in dollars per share)	$ 61		$ 53
Other Assets | Weighted Average | Income Approach Valuation Technique | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Quotes from third party pricing (in dollars per share)	$ 56.5		$ 50

[1] (1)Discounted cash flow is used as valuation technique for all financial instruments.