Investments - Equity securities reported at fair value (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	$ 1,930	$ 1,814
Common stocks
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,162	1,241
Equity securities	1,218	1,148
Equity securities, fair value in excess of cost	56	(93)
Perpetual preferred stocks
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	719	705
Equity securities	712	666
Equity securities, fair value in excess of cost	(7)	(39)
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,881	1,946
Equity securities	1,930	1,814
Equity securities, fair value in excess of cost	$ 49	$ (132)