Fair Value Measurement - Quantitative Information - Assets (Details) - Income Approach Valuation Technique [Member] - Level 3 [Member] - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Obligations of state and political subdivisions [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 82	$ 39
Obligations of state and political subdivisions [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.30%	4.30%
Obligations of state and political subdivisions [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	39.00%	22.80%
Obligations of state and political subdivisions [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	15.90%	11.10%
Corporate securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 66	$ 60
Yield (as a percent)	20.80%	20.10%
RMBS [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 346	$ 365
RMBS [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.80%	3.30%
Conditional prepayment rate (as a percent)	1.20%	1.60%
Conditional default rate (as a percent)	2.00%	1.50%
Loss severity rate (as a percent)	40.00%	30.00%
RMBS [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	9.00%	9.70%
Conditional prepayment rate (as a percent)	17.40%	17.00%
Conditional default rate (as a percent)	8.50%	10.10%
Loss severity rate (as a percent)	100.00%	100.00%
RMBS [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	5.70%	6.00%
Conditional prepayment rate (as a percent)	6.00%	4.60%
Conditional default rate (as a percent)	6.10%	6.70%
Loss severity rate (as a percent)	78.50%	77.80%
Triple-X Life Insurance Transaction [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 609	$ 425
Triple-X Life Insurance Transaction [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.10%	5.70%
Triple-X Life Insurance Transaction [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.40%	6.00%
Triple-X Life Insurance Transaction [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.30%	5.80%
Collateralized Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value		$ 332
Discount factor (as a percent)		10.00%
Collateralized loan obligations (CLO) /TruPS [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 104	$ 19
Collateralized loan obligations (CLO) /TruPS [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.50%	1.50%
Collateralized loan obligations (CLO) /TruPS [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.50%	4.80%
Collateralized loan obligations (CLO) /TruPS [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.30%	3.10%
Other Asset Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 59	$ 29
Yield (as a percent)	10.70%	7.20%
Financial Guaranty Variable Interest Entities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 707	$ 876
Financial Guaranty Variable Interest Entities [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.60%	2.90%
Conditional prepayment rate (as a percent)	3.50%	3.50%
Conditional default rate (as a percent)	1.60%	2.50%
Loss severity rate (as a percent)	55.00%	35.00%
Financial Guaranty Variable Interest Entities [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	14.90%	20.00%
Conditional prepayment rate (as a percent)	13.00%	12.00%
Conditional default rate (as a percent)	22.20%	21.60%
Loss severity rate (as a percent)	100.00%	100.00%
Financial Guaranty Variable Interest Entities [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.50%	6.50%
Conditional prepayment rate (as a percent)	9.30%	7.80%
Conditional default rate (as a percent)	4.30%	5.70%
Loss severity rate (as a percent)	78.40%	78.60%
Other Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 58	$ 62
Fair Value Inputs Term	10 years	10 years
Other Assets [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.80%	4.50%
Other Assets [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	5.50%	5.10%
Other Assets [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	5.10%	4.80%
Other invested assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 7	$ 8