Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Liabilities for Unpaid Losses and Loss Adjustment Expense (Details) - USD ($) $ in Millions	9 Months Ended		16 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	May 23, 2019	Dec. 31, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	$ 36,517
Provision for unpaid losses and loss adjustment expense
Deferred Revenue	97	$ 0
Less: payments
Ending liabilities for unpaid losses and loss adjustment expenses, gross	37,306		$ 37,306
Continental Europe Operations [Member]
Less: payments
Disposal Group, Including Discontinued Operation, Unpaid losses and loss adjustment expenses				$ 74
Adverse Development Cover Navigators Group [Member]
Provision for unpaid losses and loss adjustment expense
Prior accident year development [1]			304
Property, Liability and Casualty Insurance Product Line
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	28,261	24,584
Reinsurance and other recoverables				5,421	$ 5,275	$ 5,083		$ 4,232
Beginning liabilities for unpaid losses and loss adjustment expenses, net	23,730	22,814	23,730	23,730	$ 22,986	22,814		$ 20,352
Provision for unpaid losses and loss adjustment expense
Current accident year	5,992	5,448
Prior accident year development [1]	(320)	(23)
Total provision for unpaid losses and loss adjustment expenses	5,672	5,425
Less: payments
Current accident year	1,447	1,549
Prior accident years	3,338	3,403
Total payments	4,785	4,952
Foreign currency adjustment	(3)	(12)
Ending liabilities for unpaid losses and loss adjustment expenses, net	23,730	22,814	23,730
Ending liabilities for unpaid losses and loss adjustment expenses, gross	29,151	27,897	$ 29,151
Property, Liability and Casualty Insurance Product Line | Continental Europe Operations [Member]
Less: payments
Disposal Group, Including Discontinued Operation, Unpaid losses and loss adjustment expenses				(43)		$ 0
Property, Liability and Casualty Insurance Product Line | Adverse Development Cover Navigators Group [Member]
Provision for unpaid losses and loss adjustment expense
Deferred Revenue	$ 97	$ 0
The Navigators Group, Inc. [Member] | Property, Liability and Casualty Insurance Product Line
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Reserves net Reinsurance and Other Recoverables				$ 0			$ 2,001