Investments - Equity securities reported at fair value (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	$ 1,985	$ 1,814
Common stocks
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,163	1,241
Equity securities	1,251	1,148
Equity securities, fair value in excess of cost	88	(93)
Perpetual preferred stocks
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	731	705
Equity securities	734	666
Equity securities, fair value in excess of cost	3	(39)
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,894	1,946
Equity securities	1,985	1,814
Equity securities, fair value in excess of cost	$ 91	$ (132)