Derivative Instruments Derivative Instruments (Credit Derivatives) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	$ 1,422	$ 792
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	(28)	21
Single Name
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	100	100
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	1
Credit Default Index
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	1,322	692
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	(28)	20
NAIC 1
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	97	86
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 1 | Single Name
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	48	36
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 1 | Credit Default Index
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	49	50
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 2
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	48	60
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	1
NAIC 2 | Single Name
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	48	60
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	1
NAIC 2 | Credit Default Index
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	0	0
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 3
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	794	574
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	(8)	13
NAIC 3 | Single Name
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	4	4
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 3 | Credit Default Index
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	790	570
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	(8)	13
NAIC 4
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	0	0
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 4 | Single Name
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	0	0
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 4 | Credit Default Index
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	0	0
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 5
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	0	0
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 5 | Single Name
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	0	0
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 5 | Credit Default Index
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	0	0
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 6
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	483	72
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	(20)	7
NAIC 6 | Single Name
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	0	0
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	0	0
NAIC 6 | Credit Default Index
Derivative [Line Items]
Credit Derivative, Maximum Exposure, Undiscounted	483	72
Credit Risk Derivatives, at Fair Value, Asset Net (Liability)	$ (20)	$ 7