Investments - Mortgages by Year of Origination (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Total commercial mortgage loans	$ 4,892	$ 4,911
Year of Origination 2018
Investment [Line Items]
Total commercial mortgage loans	137	0
Year of Origination 2017
Investment [Line Items]
Total commercial mortgage loans	1,073	1,086
Year of Origination 2016
Investment [Line Items]
Total commercial mortgage loans	857	867
Year of Origination 2015
Investment [Line Items]
Total commercial mortgage loans	700	703
Year of Origination 2014
Investment [Line Items]
Total commercial mortgage loans	523	538
Year of Origination 2013
Investment [Line Items]
Total commercial mortgage loans	638	644
Year of Origination 2012 and prior
Investment [Line Items]
Total commercial mortgage loans	$ 964	$ 1,073