Investments - Equity securities reported at fair value (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	$ 2,004	$ 1,814
Common stocks
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,166	1,241
Equity securities	1,261	1,148
Equity securities, fair value in excess of cost	95	(93)
Perpetual preferred stocks
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	732	705
Equity securities	743	666
Equity securities, fair value in excess of cost	11	(39)
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at cost	1,898	1,946
Equity securities	2,004	1,814
Equity securities, fair value in excess of cost	$ 106	$ (132)