Investments - Holding gains (losses) on equity securities still held (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities									$ 289	$ 287	$ (266)
Gross investment income									2,161	2,325	2,116
Equity securities
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities									(96)	277	(265)
Accounting Standards Update 2016-01 | Equity securities
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities	$ 207	$ 30	$ 202	$ (535)	$ 67	$ (15)	$ 44	$ 182	(96)	278
Accounting Standards Update 2016-01 | Equity securities still owned | Equity securities
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Net holding gains (losses) on equity securities									(46)	207	(257)
Accounting Standards Update 2016-01 | Realized gains (losses) on securities | Equity securities still owned | Equity securities
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities									(70)	169	(279)
Accounting Standards Update 2016-01 | Investment income | Equity securities still owned | Equity securities
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Gross investment income									$ 24	$ 38	$ 22