Investments (Summary of available-for-sale corproate and other bonds in gross unrealized loss position across industry 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
Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	7,080	1,100
Gross Unrealized Losses	817	$ 32
AAA | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	8
Gross Unrealized Losses	0
AA | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	134
Gross Unrealized Losses	4
A | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	608
Gross Unrealized Losses	17
BBB | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	4,987
Gross Unrealized Losses	516
Below investment grade | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,343
Gross Unrealized Losses	280
Basic Materials | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	727
Gross Unrealized Losses	78
Communications | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	349
Gross Unrealized Losses	27
Consumer, cyclical - Other | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	390
Gross Unrealized Losses	56
Consumer, non-cyclical - Other | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	401
Gross Unrealized Losses	27
Energy - Oil & Gas | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	575
Gross Unrealized Losses	191
Energy - Pipelines | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	523
Gross Unrealized Losses	112
Entertainment | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	148
Gross Unrealized Losses	23
Financial - Other | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,711
Gross Unrealized Losses	89
Financial - Real Estate/REITS | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	599
Gross Unrealized Losses	45
Industrial | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	468
Gross Unrealized Losses	55
Retail | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	150
Gross Unrealized Losses	18
Technology | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	290
Gross Unrealized Losses	30
Transportation | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	73
Gross Unrealized Losses	5
Travel & Related | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	269
Gross Unrealized Losses	37
Utilities | Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	407
Gross Unrealized Losses	$ 24