Acquisitions and Sale of Businesses - Revenues, costs and expenses, and earnings before income taxes of business disposed (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net earned premiums	$ 1,381	$ 1,442	$ 3,774	$ 3,815
Losses and loss adjustment expenses	963	944	2,441	2,359
Commissions and other underwriting expenses	406	450	1,235	1,275
Net investment income	572	588	1,584	1,710
Net earnings, including noncontrolling interests	164	143	27	678
Neon Capital Limited
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net earned premiums	42	97	174	274
Losses and loss adjustment expenses	60	66	166	172
Commissions and other underwriting expenses	20	50	90	135
Underwriting loss	(38)	(19)	(82)	(33)
Net investment income	1	2	(5)	5
Other income and expenses, net	(3)	(3)	(5)	(8)
Net earnings, including noncontrolling interests	$ (40)	$ (20)	$ (92)	$ (36)