Fair Value Measurement - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) Security	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Security	Sep. 30, 2018 USD ($)	Dec. 31, 2018	Apr. 08, 2005 USD ($) Trust
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Percentage of CDS contracts which are fair valued using minimum premium					17.00%
Recurring [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of fixed maturity securities valued using model processes | Security	133		133
Fixed maturity securities	$ 1,135,000,000		$ 1,135,000,000
Measurement Input, Default Rate [Member] | Total [Member] | Recurring [Member] | Level 3 [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed			1.50%		2.47%
Measurement Input, Default Rate [Member] | Total [Member] | Recurring [Member] | Level 3 [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed			2.13%		2.89%
AGC [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of custodial trusts | Trust						4
Share value, amount						$ 200,000,000
Maximum amount						$ 50,000,000
Other Income [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value gains (losses) on Committed Capital Securities	$ (14,000,000)	$ (1,000,000)	$ (4,000,000)	$ (3,000,000)