Sales of Businesses - Assets and liabilities disposed (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Line Items]
Cash and investments	$ 16,387			$ 16,387		$ 13,494
Recoverables from reinsurers	3,523			3,523		3,288
Prepaid reinsurance premiums	1,028			1,028		768
Agents’ balances and premiums receivable	1,492			1,492		1,229
Other assets	847			847		977
Total assets	29,942			29,942		73,710
Unpaid losses and loss adjustment expenses	10,991		$ 10,754	10,991	$ 10,754	10,392	$ 10,232
Unearned premiums	3,415			3,415		2,803
Payable to reinsurers	1,146			1,146		807
Other liabilities	2,152			2,152		1,584
Total liabilities	24,702			24,702		$ 66,921
Realized gains (losses) on subsidiaries	$ 0		(30)	$ 4	(30)
Neon Capital Limited
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Line Items]
Estimated sale proceeds, net of expenses			4
Cash and investments			461		461
Recoverables from reinsurers			198		198
Prepaid reinsurance premiums			30		30
Agents’ balances and premiums receivable			48		48
Other assets			73		73
Total assets			810		810
Unpaid losses and loss adjustment expenses			598		598
Unearned premiums			83		83
Payable to reinsurers			39		39
Other liabilities			47		47
Total liabilities			767		767
Reclassification from accumulated other comprehensive income			(9)
Net assets of businesses to be sold			34		$ 34
Realized gains (losses) on subsidiaries		$ 4	$ (30)