Investments Investments (FHLB of Boston Advance Agreements Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	$ 1,200	$ 564
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	800	300
US Treasury and Government [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	799	300
Investment in Federal Home Loan Bank Stock [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	$ 33	$ 12