Investments (Allowance rollforward) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)
Fair Value [Line Items]
Allowance for Credit Loss, Ending Balance	$ (27)
Corporate debt securities [Member]
Fair Value [Line Items]
Allowance for Credit Loss, Beginning Balance	0	[1]
Allowance for Credit Loss, Not Previously Recorded	13
Allowance for Credit Loss, Period Increase (Decrease)	(3)
Allowance for Credit Loss, Ending Balance	10
Asset-backed Securities [Member]
Fair Value [Line Items]
Allowance for Credit Loss, Beginning Balance	0	[1]
Allowance for Credit Loss, Not Previously Recorded	1
Allowance for Credit Loss, Period Increase (Decrease)	0
Allowance for Credit Loss, Ending Balance	1
Total available-for-sale securities [Member]
Fair Value [Line Items]
Allowance for Credit Loss, Beginning Balance	0	[1]
Allowance for Credit Loss, Not Previously Recorded	14
Allowance for Credit Loss, Period Increase (Decrease)	(3)
Allowance for Credit Loss, Ending Balance	$ 11

[1] Prior to January 1, 2020, credit losses on Available-for-Sale securities were not recorded in an allowance but were recorded as a reduction of the book value of the security if the security was other-than-temporarily impaired. There is no adoption impact due to the prospective transition for Available-for-Sale securities.