Investments - Commerical Mortgage Loans by Property Type and Geographic Region (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	$ 1,761	$ 1,666
Commercial Mortgage Loans [Member]
Mortgage Loans on Real Estate [Line Items]
Commitments to make additional investments	21
Pacific Region [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	841	714
South Atlantic Region [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	210	268
New England Region [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	238	227
Central Region [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	237	239
Middle Atlantic [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	203	186
Mountain [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	32	32
Office Buildings [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	652	592
Apartment Buildings [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	608	428
Industrial [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	197	302
Hotels [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	141	205
Retail Facilities [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	135	139
Other Property [Member]
Mortgage Loans on Real Estate [Line Items]
Commercial Mortgage Loans	$ 28	$ 0