Investments (Summary of available-for-sale commercial mortgage-backed securities in gross unrealized loss position by property type and rating) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	$ 10,153	$ 2,420
Gross Unrealized Losses	1,117	45
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,197	401
Gross Unrealized Losses	153	$ 5
Conduits (multi property, multi borrower pools) | Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	211
Gross Unrealized Losses	11
Single asset, single borrower | Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	986
Gross Unrealized Losses	142
US Government, Government agencies and Government sponsored enterprises | Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1
Gross Unrealized Losses	0
AAA | Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	60
Gross Unrealized Losses	1
AA | Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	245
Gross Unrealized Losses	17
A | Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	214
Gross Unrealized Losses	22
BBB | Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	484
Gross Unrealized Losses	78
Below investment grade | Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	193
Gross Unrealized Losses	$ 35