- Quantitative Information - Assets (Details) - Valuation, Income Approach [Member] - Level 3 [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Obligations of state and political subdivisions [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 99	$ 76
Obligations of state and political subdivisions [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.50%	4.50%
Obligations of state and political subdivisions [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	32.70%	40.80%
Obligations of state and political subdivisions [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	12.00%	12.50%
Corporate securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 56	$ 67
Yield (as a percent)	29.50%	22.50%
Residential Mortgage-Backed Securities (RMBS) [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 309	$ 334
Residential Mortgage-Backed Securities (RMBS) [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	5.30%	4.00%
Residential Mortgage-Backed Securities (RMBS) [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	8.10%	7.50%
Residential Mortgage-Backed Securities (RMBS) [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.30%	5.60%
Triple-X Life Insurance Transaction [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 620	$ 613
Triple-X Life Insurance Transaction [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.50%	6.20%
Triple-X Life Insurance Transaction [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	7.10%	6.40%
Triple-X Life Insurance Transaction [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.80%	6.30%
Collateralized Loan Obligations And TruPS [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 274	$ 116
Collateralized Loan Obligations And TruPS [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	3.80%	2.60%
Collateralized Loan Obligations And TruPS [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.70%	4.60%
Collateralized Loan Obligations And TruPS [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.30%	3.30%
Other Asset Backed Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 53	$ 58
Yield (as a percent)	11.50%	10.70%
Financial Guarantee Variable Interest Entities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 569	$ 700
Financial Guarantee Variable Interest Entities [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	5.00%	3.70%
Financial Guarantee Variable Interest Entities [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	10.20%	10.00%
Financial Guarantee Variable Interest Entities [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	7.10%	6.20%
Other Assets [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 74	$ 60
Fair Value Inputs Term	10 years	10 years
Other Assets [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.60%	5.20%
Other Assets [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	7.20%	5.90%
Other Assets [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.90%	5.50%
Short-term Investments [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value		$ 1
Other invested assets [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 7	$ 7
Measurement Input, Prepayment Rate [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	3.40%	1.30%
Measurement Input, Prepayment Rate [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	19.40%	17.40%
Measurement Input, Prepayment Rate [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	6.20%	6.40%
Measurement Input, Prepayment Rate [Member] | Financial Guarantee Variable Interest Entities [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	0.90%	3.00%
Measurement Input, Prepayment Rate [Member] | Financial Guarantee Variable Interest Entities [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	18.10%	14.90%
Measurement Input, Prepayment Rate [Member] | Financial Guarantee Variable Interest Entities [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	9.30%	9.50%
Measurement Input, Default Rate [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	1.50%	1.50%
Measurement Input, Default Rate [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	6.90%	9.20%
Measurement Input, Default Rate [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	5.20%	5.90%
Measurement Input, Default Rate [Member] | Financial Guarantee Variable Interest Entities [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	1.30%	1.30%
Measurement Input, Default Rate [Member] | Financial Guarantee Variable Interest Entities [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	23.70%	21.70%
Measurement Input, Default Rate [Member] | Financial Guarantee Variable Interest Entities [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	5.10%	5.40%
Measurement Input, Loss Severity [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	40.00%	40.00%
Measurement Input, Loss Severity [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	125.00%	125.00%
Measurement Input, Loss Severity [Member] | Residential Mortgage-Backed Securities (RMBS) [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	82.70%	82.50%
Measurement Input, Loss Severity [Member] | Financial Guarantee Variable Interest Entities [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	60.00%	60.00%
Measurement Input, Loss Severity [Member] | Financial Guarantee Variable Interest Entities [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	100.00%	100.00%
Measurement Input, Loss Severity [Member] | Financial Guarantee Variable Interest Entities [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	79.80%	79.60%
Financial Guarantee Variable Interest Entities [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	5.00%	3.40%
Financial Guarantee Variable Interest Entities [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	10.20%	10.00%
Financial Guarantee Variable Interest Entities [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	5.60%	4.90%
Financial Guarantee Variable Interest Entities [Member] | Measurement Input, Prepayment Rate [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	0.90%	3.00%
Financial Guarantee Variable Interest Entities [Member] | Measurement Input, Prepayment Rate [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	18.10%	14.90%
Financial Guarantee Variable Interest Entities [Member] | Measurement Input, Prepayment Rate [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	9.30%	9.50%
Financial Guarantee Variable Interest Entities [Member] | Measurement Input, Default Rate [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	1.30%	1.30%
Financial Guarantee Variable Interest Entities [Member] | Measurement Input, Default Rate [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	23.70%	21.70%
Financial Guarantee Variable Interest Entities [Member] | Measurement Input, Default Rate [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	5.10%	5.40%
Financial Guarantee Variable Interest Entities [Member] | Measurement Input, Loss Severity [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	60.00%	60.00%
Financial Guarantee Variable Interest Entities [Member] | Measurement Input, Loss Severity [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	100.00%	100.00%
Financial Guarantee Variable Interest Entities [Member] | Measurement Input, Loss Severity [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate (as a percent)	79.80%	79.60%