Investments - Equity securities reported at fair value (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Equity securities, at fair value	$ 1,777	$ 1,662
Common stocks
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,066
Equity securities, at fair value	1,180
Equity securities, fair value in excess of cost	114
Perpetual preferred stocks
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	603
Equity securities, at fair value	597
Equity securities, fair value in excess of cost	(6)
Equity securities
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,669
Equity securities, at fair value	1,777
Equity securities, fair value in excess of cost	$ 108