Investments (Net investment income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net Investment Income [Line Items]
Gross investment income	$ 522	$ 490	$ 1,027	$ 1,049
Investment expense	(16)	(15)	(31)	(29)
Net investment income	506	475	996	1,020
Fixed maturity securities
Net Investment Income [Line Items]
Gross investment income	444	457	890	912
Equity securities
Net Investment Income [Line Items]
Gross investment income	12	2	22	3
Limited partnership investments
Net Investment Income [Line Items]
Gross investment income	40	16	70	106
Mortgage loans
Net Investment Income [Line Items]
Gross investment income	14	8	25	15
Short term investments
Net Investment Income [Line Items]
Gross investment income	6	3	12	6
Trading portfolio
Net Investment Income [Line Items]
Gross investment income	3	4	5	6
Other
Net Investment Income [Line Items]
Gross investment income	$ 3	$ 0	$ 3	$ 1