Investments - Holding gains (losses) on equity securities still held (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities still owned	$ 34	$ (12)	$ (28)	$ (1)
Included in net investment income	533	475	1,568	1,379
Equity securities
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities still owned	33	$ (10)	(39)	$ (20)
Equity securities still owned | Equity securities | Accounting Standards Update 2016-01
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Net holding gains (losses) on equity securities	27		(35)
Equity securities still owned | Realized gains (losses) on securities | Equity securities | Accounting Standards Update 2016-01
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities still owned	25		(51)
Equity securities still owned | Investment income | Equity securities | Accounting Standards Update 2016-01
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Included in net investment income	$ 2		$ 16