Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Liabilities for Unpaid Losses and Loss Adjustment Expense (Details) - USD ($) $ in Millions	3 Months Ended		22 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	$ 37,855
Less: payments
Ending liabilities for unpaid losses and loss adjustment expenses, gross	38,540		$ 38,540
Continental Europe Operations [Member]
Less: payments
Disposal Group, Including Discontinued Operation, Unpaid losses and loss adjustment expenses				$ (77)	$ (84)
Adverse Development Cover Navigators Group [Member]
Provision for unpaid losses and loss adjustment expense
Prior accident year development [1]			315
Deferred Revenue	6	$ 29
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	29,622	28,261
Reinsurance and other recoverables				5,808	5,725	$ 5,332	$ 5,275
Beginning liabilities for unpaid losses and loss adjustment expenses, net	24,524	23,048	24,524	24,524	$ 23,897	23,048	$ 22,986
Provision for unpaid losses and loss adjustment expense
Current accident year	1,924	1,883
Prior accident year development [1]	229	23
Total provision for unpaid losses and loss adjustment expenses	2,153	1,906
Less: payments
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	(292)	(304)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	(1,221)	(1,491)
Total payments	(1,513)	(1,795)
Foreign currency adjustment	(6)	(20)
Ending liabilities for unpaid losses and loss adjustment expenses, net	24,524	23,048	24,524
Ending liabilities for unpaid losses and loss adjustment expenses, gross	30,332	28,380	$ 30,332
Property, Liability and Casualty Insurance Product Line | Continental Europe Operations [Member]
Less: payments
Disposal Group, Including Discontinued Operation, Unpaid losses and loss adjustment expenses				$ 1		$ 0
Property, Liability and Casualty Insurance Product Line | Adverse Development Cover Navigators Group [Member]
Provision for unpaid losses and loss adjustment expense
Deferred Revenue	$ 6	$ 29