Investments - Holding gains (losses) on equity securities still held (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities		$ (266)	$ 5	$ 19
Gross investment income		2,116	1,852	1,716
Equity securities
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities		(265)	$ 6	$ 13
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		(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		(279)
Net holding gains (losses) on equity securities	$ (228)	(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		$ 22