Investments - Recorded Investment in CMLs by LTV and DSC Ratio Categories (Details) - Commercial Portfolio Segment $ in Millions	Sep. 30, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 546
% of Total	100.00%
Fair Value
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 560
% of Total	100.00%
Greater than 1.25
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 539
Greater than 1.00 but less than 1.25
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	7
Less than 50%
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 420
% of Total	77.00%
Less than 50% | Fair Value
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 430
% of Total	77.00%
Less than 50% | Greater than 1.25
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 413
Less than 50% | Greater than 1.00 but less than 1.25
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	7
LTV 50 to 60 Percent
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 94
% of Total	17.00%
LTV 50 to 60 Percent | Fair Value
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 96
% of Total	17.00%
LTV 50 to 60 Percent | Greater than 1.25
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 94
LTV 50 to 60 Percent | Greater than 1.00 but less than 1.25
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	0
LTV 60 to 75 Percent
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 32
% of Total	6.00%
LTV 60 to 75 Percent | Fair Value
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 34
% of Total	6.00%
LTV 60 to 75 Percent | Greater than 1.25
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 32
LTV 60 to 75 Percent | Greater than 1.00 but less than 1.25
Debt Securities, Available-for-sale [Line Items]
Unpaid Principal Balance	$ 0