Fair Value Measurement - Quantitative Information - Assets (Details) - Valuation, Income Approach [Member] - Level 3 [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Obligations of state and political subdivisions [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 104	$ 99
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%	32.70%
Obligations of state and political subdivisions [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	10.10%	12.00%
Corporate securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 48	$ 56
Yield (as a percent)	29.10%	29.50%
RMBS [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 318	$ 309
RMBS [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.70%	5.30%
RMBS [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	7.20%	8.10%
RMBS [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	5.50%	6.30%
Life Insurance Transaction [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 620	$ 620
Life Insurance Transaction [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.00%	6.50%
Life Insurance Transaction [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.60%	7.10%
Life Insurance Transaction [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.30%	6.80%
Collateralized loan obligations (CLO) /TruPS [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 286	$ 274
Collateralized loan obligations (CLO) /TruPS [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	3.20%	3.80%
Collateralized loan obligations (CLO) /TruPS [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.80%	4.70%
Collateralized loan obligations (CLO) /TruPS [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.00%	4.30%
Other Asset Backed Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 52	$ 53
Yield (as a percent)	10.70%	11.50%
FG VIEs and other [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 560	$ 569
FG VIEs and other [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.40%	5.00%
FG VIEs and other [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	9.50%	10.20%
FG VIEs and other [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.30%	7.10%
Other Assets [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 65	$ 74
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)	6.30%	6.60%
Other Assets [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.90%	7.20%
Other Assets [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.60%	6.90%
Other invested assets [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Asset, fair value	$ 7	$ 7
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	2.70%	3.40%
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	15.80%	19.40%
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	6.10%	6.20%
Measurement Input, Prepayment Rate [Member] | FG VIEs and other [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%	0.90%
Measurement Input, Prepayment Rate [Member] | FG VIEs and other [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	18.30%	18.10%
Measurement Input, Prepayment Rate [Member] | FG VIEs and other [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	9.10%	9.30%
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	6.90%	6.90%
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	5.10%	5.20%
Measurement Input, Default Rate [Member] | FG VIEs and other [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] | FG VIEs and other [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	24.00%	23.70%
Measurement Input, Default Rate [Member] | FG VIEs and other [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	4.90%	5.10%
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	85.40%	82.70%
Measurement Input, Loss Severity [Member] | FG VIEs and other [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] | FG VIEs and other [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] | FG VIEs and other [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	79.70%	79.80%