Commitments and Contingencies - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
Loss Contingencies [Line Items]
Pledged Collateral	$ 1,508	$ 1,238
Acquisition of mortgage loans
Loss Contingencies [Line Items]
Amount of purchase commitment	273
Purchase of limited partnerships and private placement investments
Loss Contingencies [Line Items]
Amount of purchase commitment	2,060
Federal Home Loan Bank of Boston | Line of Credit
Loss Contingencies [Line Items]
Non-putable funding agreements issued to FHLB	$ 973	$ 721