Derivative Instruments - Derivatives - Credit Risk Assumed through Credit Derivatives (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Credit Derivatives [Line Items]
Fair Value	$ (56)	$ (66)
Derivative, Notional Amount	11,854	13,706
Credit Default Swap, Selling Protection [Member]
Credit Derivatives [Line Items]
Derivative, Notional Amount	263	515
Credit derivatives
Credit Derivatives [Line Items]
Fair Value	(6)	8
Offsetting Notional Amount	13	15
Offsetting Fair Value	4	5
Credit derivatives | Single name credit default swaps | A Minus | Investment grade risk exposure
Credit Derivatives [Line Items]
Fair Value	$ (1)	$ 3
Weighted Average Years to Maturity	5 years	5 years
Offsetting Notional Amount	$ 0	$ 0
Offsetting Fair Value	0	0
Derivative, Notional Amount	100	100
Credit derivatives | Basket credit default swaps | BBB Plus | Investment grade risk exposure
Credit Derivatives [Line Items]
Fair Value	$ (1)	$ 10
Weighted Average Years to Maturity	5 years	5 years
Offsetting Notional Amount	$ 0	$ 0
Offsetting Fair Value	0	0
Derivative, Notional Amount	150	400
Credit derivatives | Basket credit default swaps | A | Investment grade risk exposure
Credit Derivatives [Line Items]
Fair Value		$ 0
Average Term of Credit Risk Derivatives Less than 1 Year		Less than 1 year
Offsetting Notional Amount		$ 1
Offsetting Fair Value		0
Derivative, Notional Amount		1
Credit derivatives | Basket credit default swaps | CCC Minus | Below investment grade risk exposure
Credit Derivatives [Line Items]
Fair Value	$ (4)	$ (5)
Average Term of Credit Risk Derivatives Less than 1 Year	Less than 1 year	Less than 1 year
Offsetting Notional Amount	$ 13	$ 14
Offsetting Fair Value	4	5
Derivative, Notional Amount	$ 13	$ 14