Fair Value Measurement - Quantitative Information - Assets (Details) - Valuation, Income Approach [Member] - Level 3 [Member] - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 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)	29.70%	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)	9.10%	12.50%
Corporate securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 60	$ 67
Yield (as a percent)	27.30%	22.50%
RMBS [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 299	$ 334
RMBS [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	5.30%	4.00%
RMBS [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	7.90%	7.50%
RMBS [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.20%	5.60%
Triple-X Life Insurance Transaction [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 642	$ 613
Triple-X Life Insurance Transaction [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.60%	6.20%
Triple-X Life Insurance Transaction [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.70%	6.40%
Triple-X Life Insurance Transaction [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.70%	6.30%
Collateralized loan obligations (CLO) /TruPS [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 256	$ 116
Collateralized loan obligations (CLO) /TruPS [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	3.50%	2.60%
Collateralized loan obligations (CLO) /TruPS [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.70%	4.60%
Collateralized loan obligations (CLO) /TruPS [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.20%	3.30%
Other Asset Backed Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 55	$ 58
Yield (as a percent)	11.20%	10.70%
Financial Guaranty Variable Interest Entities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 596	$ 700
Financial Guaranty Variable Interest Entities [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.50%	3.70%
Financial Guaranty Variable Interest Entities [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	10.30%	10.00%
Financial Guaranty Variable Interest Entities [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.90%	6.20%
Other Assets [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 57	$ 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)	5.90%	5.20%
Other Assets [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.50%	5.90%
Other Assets [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.20%	5.50%
Other invested assets [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 7	$ 7
Short-term Investments [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value		$ 1
Measurement Input, Prepayment Rate [Member] | RMBS [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	2.80%	1.30%
Measurement Input, Prepayment Rate [Member] | RMBS [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	18.00%	17.40%
Measurement Input, Prepayment Rate [Member] | RMBS [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	6.30%	6.40%
Measurement Input, Prepayment Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	0.90%	3.00%
Measurement Input, Prepayment Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	17.50%	14.90%
Measurement Input, Prepayment Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	9.30%	9.50%
Measurement Input, Default Rate [Member] | RMBS [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	1.50%	1.50%
Measurement Input, Default Rate [Member] | RMBS [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	8.80%	9.20%
Measurement Input, Default Rate [Member] | RMBS [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	5.40%	5.90%
Measurement Input, Default Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	1.30%	1.30%
Measurement Input, Default Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	23.50%	21.70%
Measurement Input, Default Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	5.60%	5.40%
Measurement Input, Loss Severity [Member] | RMBS [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	40.00%	40.00%
Measurement Input, Loss Severity [Member] | RMBS [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	125.00%	125.00%
Measurement Input, Loss Severity [Member] | RMBS [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	82.20%	82.50%
Measurement Input, Loss Severity [Member] | Financial Guaranty Variable Interest Entities [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	60.00%	60.00%
Measurement Input, Loss Severity [Member] | Financial Guaranty Variable Interest Entities [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	100.00%	100.00%
Measurement Input, Loss Severity [Member] | Financial Guaranty Variable Interest Entities [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	79.80%	79.60%