Investments Investments (FHLB Boston Advance Agreements Remaining Tenor) (Details) - USD ($) $ in Millions	Mar. 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
Foreign Government Debt Securities [Member] | Maturity 30 to 180 Days [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	625	300
Foreign Government Debt Securities [Member] | Maturity 180 to 360 Days [Member] [Domain]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	75	0
Foreign Government Debt Securities [Member] | Maturity Less than 30 Days [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	$ 100	$ 0