- Quantitative Information - Assets (Details) - Income Approach Valuation Technique [Member] - Level 3 [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Obligations of state and political subdivisions [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 76	$ 39
Obligations of state and political subdivisions [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.50%	4.30%
Obligations of state and political subdivisions [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	40.80%	22.80%
Obligations of state and political subdivisions [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	12.50%	11.10%
Corporate securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 67	$ 60
Yield (as a percent)	22.50%	20.10%
Residential Mortgage-Backed Securities (RMBS) [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 334	$ 365
Residential Mortgage-Backed Securities (RMBS) [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.00%	3.30%
Conditional prepayment rate (as a percent)	1.30%	1.60%
Conditional default rate (as a percent)	1.50%	1.50%
Loss severity rate (as a percent)	40.00%	30.00%
Residential Mortgage-Backed Securities (RMBS) [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	7.50%	9.70%
Conditional prepayment rate (as a percent)	17.40%	17.00%
Conditional default rate (as a percent)	9.20%	10.10%
Loss severity rate (as a percent)	125.00%	100.00%
Residential Mortgage-Backed Securities (RMBS) [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	5.60%	6.00%
Conditional prepayment rate (as a percent)	6.40%	4.60%
Conditional default rate (as a percent)	5.90%	6.70%
Loss severity rate (as a percent)	82.50%	77.80%
Triple-X Life Insurance Transaction [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 613	$ 425
Triple-X Life Insurance Transaction [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.20%	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%
Pooled corporate obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value		$ 332
Yield (as a percent)		10.00%
Collateralized Loan Obligations And TruPS [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 116	$ 19
Collateralized Loan Obligations And TruPS [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.60%	1.50%
Collateralized Loan Obligations And TruPS [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.60%	4.80%
Collateralized Loan Obligations And 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	$ 58	$ 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	$ 700	$ 876
Financial Guaranty Variable Interest Entities [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.70%	2.90%
Conditional prepayment rate (as a percent)	3.00%	3.50%
Conditional default rate (as a percent)	1.30%	2.50%
Loss severity rate (as a percent)	60.00%	35.00%
Financial Guaranty Variable Interest Entities [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	10.00%	20.00%
Conditional prepayment rate (as a percent)	14.90%	12.00%
Conditional default rate (as a percent)	21.70%	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)	6.20%	6.50%
Conditional prepayment rate (as a percent)	9.50%	7.80%
Conditional default rate (as a percent)	5.40%	5.70%
Loss severity rate (as a percent)	79.60%	78.60%
Other Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 60	$ 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)	5.20%	4.50%
Other Assets [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	5.90%	5.10%
Other Assets [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	5.50%	4.80%
Short-term Investments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 1
Other invested assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 7	$ 8