Commitments and Contingencies - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Loss Contingencies [Line Items]
Operating Lease, Expense	$ 2	$ 3	$ 3
Loss Contingency, Range of Possible Loss, Portion Not Accrued	0
Loan Purchase Commitments
Loss Contingencies [Line Items]
Long-term Purchase Commitment, Amount	56
Investment Purchase Commitment
Loss Contingencies [Line Items]
Long-term Purchase Commitment, Amount	689
Federal Home Loan Bank of Boston | Line of Credit
Loss Contingencies [Line Items]
Line of Credit Facility, Fair Value of Amount Outstanding	$ 671	$ 730