Investments - Equity securities reported at fair value (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	$ 1,827	$ 1,662
Common stocks
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,040
Equity securities	1,151
Equity securities, fair value in excess of cost	111
Perpetual preferred stocks
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	683
Equity securities	676
Equity securities, fair value in excess of cost	(7)
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,723
Equity securities	1,827
Equity securities, fair value in excess of cost	$ 104