Investments - Net investment income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Net Investment Income:
Short-term investments	$ 3,424		$ 1,944
Investment income, before expense	851	$ 799
Investment expense	(65)	(51)
Net investment income	786	748
Valuation of equity investments	(83)	0
Fixed income securities
Net Investment Income:
Investment income, before expense	508	518
Equity securities
Net Investment Income:
Investment income, before expense	34	44
Mortgage loans
Net Investment Income:
Investment income, before expense	51	55
Limited partnership interests
Net Investment Income:
Investment income, before expense	180	120
Valuation of equity investments	77
Short-term investments
Net Investment Income:
Investment income, before expense	12	6
Other
Net Investment Income:
Investment income, before expense	66	$ 56
EMA limited partnerships | Limited partnership interests
Net Investment Income:
Investment income, before expense	$ 103