Acquisitions and Sale of Businesses - Assets and liabilities disposed (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and investments		$ 13,494	$ 15,745	$ 13,494
Recoverables from reinsurers		3,288	3,519	3,288
Prepaid reinsurance premiums		768	834	768
Agents’ balances and premiums receivable		1,229	1,265	1,229
Other assets		977	902	977
Total assets		73,710	28,931	73,710
Unpaid losses and loss adjustment expenses		10,392	11,074	10,392	$ 10,232	$ 9,741
Unearned premiums		2,803	3,041	2,803
Payable to reinsurers		807	920	807
Other liabilities		1,584	1,700	1,584
Total liabilities		66,921	23,919	66,921
Realized gains on subsidiaries			$ 4	23	$ 0
Neon Capital Limited
Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale proceeds, net of expenses				3
Cash and investments		453		453
Recoverables from reinsurers		224		224
Prepaid reinsurance premiums		8		8
Agents’ balances and premiums receivable		42		42
Other assets		60		60
Total assets		787		787
Unpaid losses and loss adjustment expenses		640		640
Unearned premiums		49		49
Payable to reinsurers		19		19
Other liabilities		92		92
Total liabilities		800		800
Reclassification from accumulated other comprehensive income				(7)
Net liabilities of businesses sold		(20)		(20)
Realized gains on subsidiaries	$ 4	$ 23		$ 23