Investments - Holding gains (losses) on equity securities still held (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities	$ 287	$ (266)	$ 5
Gross investment income	2,325	2,116	1,852
Equity securities
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities	277	(265)	$ 6
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	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	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	$ 38	$ 22