Acquisitions and Sale of Businesses - Assets and liabilities disposed (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and investments	$ 58,087		$ 58,087		$ 55,252
Recoverables from reinsurers	3,819		3,819		3,415
Prepaid reinsurance premiums	862		862		678
Agents’ balances and premiums receivable	1,384		1,384		1,335
Other assets	1,749		1,749		1,867
Total assets	73,110		73,110		70,130
Unpaid losses and loss adjustment expenses	10,754	$ 9,847	10,754	$ 9,847	10,232	$ 9,741
Unearned premiums	3,015		3,015		2,830
Payable to reinsurers	977		977		814
Other liabilities	2,231		2,231		2,295
Total liabilities	66,770		66,770		$ 63,861
Realized losses on subsidiaries	(30)	$ 0	(30)	$ 0
Neon Capital Limited
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [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 business to be sold	34		$ 34
Realized losses on subsidiaries	$ (30)