Investments (Net investment income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net Investment Income [Line Items]
Gross investment income	$ 501	$ 524	$ 1,528	$ 1,573
Investment expense	(14)	(15)	(45)	(44)
Net investment income	487	509	1,483	1,529
Fixed maturity securities
Net Investment Income [Line Items]
Gross investment income	449	455	1,339	1,367
Equity securities
Net Investment Income [Line Items]
Gross investment income	10	1	32	4
Limited partnership investments
Net Investment Income [Line Items]
Gross investment income	23	51	93	157
Mortgage loans
Net Investment Income [Line Items]
Gross investment income	11	9	36	24
Short term investments
Net Investment Income [Line Items]
Gross investment income	6	4	18	10
Trading portfolio
Net Investment Income [Line Items]
Gross investment income	1	3	6	9
Other
Net Investment Income [Line Items]
Gross investment income	$ 1	$ 1	$ 4	$ 2