Investments - Mortgages by Year of Origination (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Total Commercial mortgage loans	$ 4,919	$ 4,911
Year of Origination 2018
Investment [Line Items]
Total Commercial mortgage loans	375	0
Year of Origination 2017
Investment [Line Items]
Total Commercial mortgage loans	1,108	1,086
Year of Origination 2016
Investment [Line Items]
Total Commercial mortgage loans	906	867
Year of Origination 2015
Investment [Line Items]
Total Commercial mortgage loans	589	703
Year of Origination 2014
Investment [Line Items]
Total Commercial mortgage loans	490	538
Year of Origination 2013
Investment [Line Items]
Total Commercial mortgage loans	585	644
Year of Origination 2012
Investment [Line Items]
Total Commercial mortgage loans	$ 866	$ 1,073