Investments - Equity securities reported at fair value (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Equity securities, at fair value	$ 1,786	$ 1,662
Common stocks
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,099
Equity securities, at fair value	1,154
Equity securities, fair value in excess of cost	55
Perpetual preferred stocks
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	632
Equity securities, at fair value	632
Equity securities, fair value in excess of cost	0
Equity securities
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,731
Equity securities, at fair value	1,786
Equity securities, fair value in excess of cost	$ 55