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
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	$ 42,318
Less: payments
Ending liabilities for unpaid losses and loss adjustment expenses, gross	42,771
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group
Less: payments
Amortization of Deferred Gain on Retroactive Reinsurance	24
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group | Maximum
Less: payments
Reinsurance, Excess Retention, Amount Reinsured, Per Policy	300
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group | Retention Layer for Reserve for the Covered Liabilities as of the Inception Date
Less: payments
Reinsurance, Amount Retained, Per Policy	1,816
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group | Retention Layer Above Reserve for the Covered Liabilities as of the Inception Date
Less: payments
Reinsurance, Amount Retained, Per Policy	100
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	34,044	$ 33,083
Reinsurance and other recoverables	6,705	6,469	$ 6,696	$ 6,465
Liability for Unpaid Claims and Claims Adjustment Expense, Net	27,832	26,929	27,348	26,618
Provision for unpaid losses and loss adjustment expense
Current accident year	2,461	2,270
Prior accident year development [1]	(56)	0
Total provision for unpaid losses and loss adjustment expenses	2,405	2,270
Less: payments
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	(349)	(321)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	(1,591)	(1,646)
Total payments	(1,940)	(1,967)
Foreign currency adjustment	(5)	8
Ending liabilities for unpaid losses and loss adjustment expenses, net	27,832	26,929
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	(6,705)	(6,469)	$ (6,696)	$ (6,465)
Ending liabilities for unpaid losses and loss adjustment expenses, gross	$ 34,537	$ 33,398