Investments, Available-for-Sale Mortgage-Backed Securities in Gross Unrealized Loss Position (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Commercial Mortgage-Backed [Member]
Available-for-sale Securities [Abstract]
Gross unrealized losses	$ 153	$ 5
Estimated fair value	1,197	$ 401
Commercial Mortgage-Backed [Member] | AAA [Member]
Available-for-sale Securities [Abstract]
Gross unrealized losses	1
Estimated fair value	60
Commercial Mortgage-Backed [Member] | AA [Member]
Available-for-sale Securities [Abstract]
Gross unrealized losses	17
Estimated fair value	245
Commercial Mortgage-Backed [Member] | A [Member]
Available-for-sale Securities [Abstract]
Gross unrealized losses	22
Estimated fair value	214
Commercial Mortgage-Backed [Member] | BBB [ Member]
Available-for-sale Securities [Abstract]
Gross unrealized losses	78
Estimated fair value	484
Commercial Mortgage-Backed [Member] | Below Investment Grade [Member]
Available-for-sale Securities [Abstract]
Gross unrealized losses	35
Estimated fair value	193
Commercial Mortgage-Backed [Member] | Single Property [Member]
Available-for-sale Securities [Abstract]
Gross unrealized losses	142
Estimated fair value	986
Commercial Mortgage-Backed [Member] | Conduit [Member]
Available-for-sale Securities [Abstract]
Gross unrealized losses	11
Estimated fair value	211
US Government, Government Agencies and Government Sponsored Enterprises [Member]
Available-for-sale Securities [Abstract]
Estimated fair value	$ 1