Acquisitions and Sale of Businesses - Revenues, costs and expenses, and earnings before income taxes of business disposed (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Sale of Businesses
Net earned premiums									$ 5,099	$ 5,185	$ 4,865
Loss and loss adjustment expenses									3,271	3,271	3,003
Commissions and other underwriting expenses									1,625	1,725	1,583
Net investment income									2,132	2,303	2,094
Net earnings, including noncontrolling interests	$ 694	$ 164	$ 167	$ (304)	$ 191	$ 143	$ 209	$ 326	721	869	517
Neon Capital Limited
Sale of Businesses
Net earned premiums									200	384	306
Loss and loss adjustment expenses									218	225	204
Commissions and other underwriting expenses									117	195	165
Underwriting loss									(135)	(36)	(63)
Net investment income									(5)	6	10
Other income and expenses, net									(5)	(10)	(5)
Net earnings, including noncontrolling interests									$ (145)	$ (40)	$ (58)