Investments - Holding gains (losses) on equity securities still held (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Holding gains (losses) on equity securities still held	$ 44	$ 95	$ 132	$ (207)
Realized gains (losses) on securities
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Holding gains (losses) on equity securities still held	36	86	98	(206)
Investment income
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Holding gains (losses) on equity securities still held	$ 8	$ 9	$ 34	$ (1)