Investments (FHLB of Boston Advance Agreements Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	$ 1,200	$ 1,200
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	800	800
U.S. government and agency
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	812	799
Investment in Federal Home Loan Bank Stock
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	$ 33	$ 33