Investments Investments (FHLB Boston Advance Agreements Remaining Tenor) (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, Branch of FHLB Bank, Amount of Advances	$ 800	$ 300
Municipal Bonds [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	800	300
Maturity Less than 30 Days [Member] | Municipal Bonds [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	150	0
Maturity 30 to 180 Days [Member] | Municipal Bonds [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	650	300
Maturity 180 to 360 Days [Member] [Domain] | Municipal Bonds [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	$ 0	$ 0