Acquisitions and Sale of Businesses - Assets and liabilities disposed (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Sale of Businesses
Cash and investments	$ 52,502	$ 52,502	$ 55,252
Prepaid reinsurance premiums	768	768	678
Agents’ balances and premiums receivable	1,231	1,231	1,335
Other assets	1,626	1,626	1,867
Total assets	73,566	73,566	70,130	$ 63,456
Unpaid losses and loss adjustment expenses	10,392	10,392	10,232	9,741	$ 9,678
Unearned premiums	2,803	2,803	2,830
Payable to reinsurers	807	807	814
Other liabilities	2,197	2,197	2,295
Total liabilities	66,777	66,777	63,861
Realized gains on subsidiaries		23	$ 0	$ 0
Neon Capital Limited
Sale of Businesses
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 assets of business to be sold	(20)	(20)
Realized gains on subsidiaries	$ 23	$ 23