Fair Value Measurement - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) Trust Security	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Trust Security	Sep. 30, 2019 USD ($)	Dec. 31, 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value gains (losses) on Committed Capital Securities	$ 10,000,000	$ 14,000,000	$ (13,000,000)	$ 4,000,000
Percentage of CDS contracts fair valued using minimum premium (de minimis)	0.00%		0.00%
Recurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of fixed maturity securities valued using model processes | Security	200		200
Fixed maturity securities	$ 1,277,000,000		$ 1,277,000,000
Measurement Input, Default Rate | Total | Recurring | Level 3 | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed			0.22%		1.69%
Measurement Input, Default Rate | Total | Recurring | Level 3 | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed			1.06%		2.08%
AGC
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of custodial trusts | Trust	4		4
Share value, amount	$ 200,000,000		$ 200,000,000
Maximum amount	$ 50,000,000		$ 50,000,000