Investments - Net appreciation (decline) recognized in net income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investment [Line Items]
Total	$ 156	$ 309	$ 845	$ 502
Equity securities
Investment [Line Items]
Total	107	234	736	321
Limited partnership interests
Investment [Line Items]
Total	$ 49	$ 75	$ 109	$ 181