Fair Value Measurement - Quantitative Information - Assets (Details) - Income Approach Valuation Technique [Member] - Level 3 [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Obligations of state and political subdivisions [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 83	$ 76
Obligations of state and political subdivisions [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.50%	4.50%
Obligations of state and political subdivisions [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	39.00%	40.80%
Obligations of state and political subdivisions [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	9.60%	12.50%
Corporate securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 62	$ 67
Yield (as a percent)	22.80%	22.50%
RMBS [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 314	$ 334
RMBS [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.70%	4.00%
Conditional prepayment rate (as a percent)	1.40%	1.30%
Conditional default rate (as a percent)	1.50%	1.50%
Loss severity rate (as a percent)	40.00%	40.00%
RMBS [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	7.80%	7.50%
Conditional prepayment rate (as a percent)	15.00%	17.40%
Conditional default rate (as a percent)	8.60%	9.20%
Loss severity rate (as a percent)	125.00%	125.00%
RMBS [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	5.90%	5.60%
Conditional prepayment rate (as a percent)	6.40%	6.40%
Conditional default rate (as a percent)	5.70%	5.90%
Loss severity rate (as a percent)	82.40%	82.50%
Triple-X Life Insurance Transaction [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 628	$ 613
Triple-X Life Insurance Transaction [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.30%	6.20%
Triple-X Life Insurance Transaction [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.90%	6.40%
Triple-X Life Insurance Transaction [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.60%	6.30%
Collateralized loan obligations (CLO) /TruPS [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 124	$ 116
Collateralized loan obligations (CLO) /TruPS [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.00%	2.60%
Collateralized loan obligations (CLO) /TruPS [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.80%	4.60%
Collateralized loan obligations (CLO) /TruPS [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.60%	3.30%
Other Asset Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 57	$ 58
Yield (as a percent)	11.30%	10.70%
Financial Guaranty Variable Interest Entities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 651	$ 700
Financial Guaranty Variable Interest Entities [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.10%	3.70%
Conditional prepayment rate (as a percent)	1.70%	3.00%
Conditional default rate (as a percent)	1.30%	1.30%
Loss severity rate (as a percent)	60.00%	60.00%
Financial Guaranty Variable Interest Entities [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	10.40%	10.00%
Conditional prepayment rate (as a percent)	17.80%	14.90%
Conditional default rate (as a percent)	21.90%	21.70%
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)	6.60%	6.20%
Conditional prepayment rate (as a percent)	9.20%	9.50%
Conditional default rate (as a percent)	5.20%	5.40%
Loss severity rate (as a percent)	79.70%	79.60%
Other Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 59	$ 60
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)	5.50%	5.20%
Other Assets [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.10%	5.90%
Other Assets [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	5.80%	5.50%
Other invested assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Asset, fair value	$ 7	$ 7