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:
Investment income, before expense	$ 890	$ 945	$ 1,741	$ 1,744
Investment expense	(66)	(48)	(131)	(99)
Net investment income	824	897	1,610	1,645
Valuation of equity investments	34	0	(49)	0
Fixed income securities
Net Investment Income:
Investment income, before expense	509	527	1,017	1,045
Equity securities
Net Investment Income:
Investment income, before expense	61	49	95	93
Mortgage loans
Net Investment Income:
Investment income, before expense	60	50	111	105
Limited partnership interests
Net Investment Income:
Investment income, before expense	173	253	353	373
Valuation of equity investments	30		107
Short-term investments
Net Investment Income:
Investment income, before expense	19	6	31	12
Other
Net Investment Income:
Investment income, before expense	68	$ 60	134	$ 116
EMA limited partnerships | Limited partnership interests
Net Investment Income:
Investment income, before expense	$ 143		$ 246