Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Liabilities for Unpaid Losses and Loss Adjustment Expense (Details) - USD ($) $ in Millions	6 Months Ended		13 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Jun. 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	83	$ 0
Less: payments
Ending liabilities for unpaid losses and loss adjustment expenses, gross	36,870		$ 36,870
Adverse Development Cover Navigators Group [Member]
Provision for unpaid losses and loss adjustment expense
Prior accident year development [1]			290
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				$ 5,427	$ 5,275	$ 5,125		$ 4,232
Beginning liabilities for unpaid losses and loss adjustment expenses, net	23,257	22,623	23,257	23,257	$ 22,986	$ 22,623		$ 20,352
Provision for unpaid losses and loss adjustment expense
Current accident year	3,962	3,475
Prior accident year development [1]	(245)	24
Total provision for unpaid losses and loss adjustment expenses	3,717	3,499
Less: payments
Current accident year	778	848
Prior accident years	2,575	2,381
Total payments	3,353	3,229
Foreign currency adjustment	(10)	0
Ending liabilities for unpaid losses and loss adjustment expenses, net	23,257	22,623	23,257
Ending liabilities for unpaid losses and loss adjustment expenses, gross	28,684	27,748	$ 28,684
Property, Liability and Casualty Insurance Product Line | Adverse Development Cover Navigators Group [Member]
Provision for unpaid losses and loss adjustment expense
Deferred Revenue	$ 83	$ 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