Investments - Holding gains (losses) on equity securities still held (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities still owned	$ 56	$ 31	$ 240	$ (62)
Included in net investment income	586	535	1,134	1,035
Equity securities
Holding Gains (Losses) on Equity Securities Still Held [Line Items]
Realized gains (losses) on securities still owned	44	23	226	(72)
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	45	31	211	(57)
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	38	16	193	(71)
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	$ 7	$ 15	$ 18	$ 14