Commitments and Contingencies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Loss Contingencies [Line Items]
Fair value of assets pledged as collateral	$ 1,111	$ 1,087
Acquisition of mortgage loans
Loss Contingencies [Line Items]
Amount of purchase commitment	77
Purchase of limited partnerships and private placement investments
Loss Contingencies [Line Items]
Amount of purchase commitment	506
Federal Home Loan Bank of Boston | Line of Credit
Loss Contingencies [Line Items]
Non-putable funding agreements issued to FHLB	856	877
Fair value of assets pledged as collateral	$ 1,111	$ 1,087