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
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	May 23, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	$ 33,029
Less: payments
Foreign currency adjustment	(12)	$ 0
Ending liabilities for unpaid losses and loss adjustment expenses, gross	36,188
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	24,584	23,775
Reinsurance and other recoverables			$ 5,083		$ 4,232	$ 3,780	$ 3,957
Beginning liabilities for unpaid losses and loss adjustment expenses, net	22,814	20,017	$ 22,814		20,352	$ 20,017	$ 19,818
Provision for unpaid losses and loss adjustment expense
Current accident year	5,448	5,151
Prior accident year development	(23)	(139)
Total provision for unpaid losses and loss adjustment expenses	5,425	5,012
Less: payments
Current accident year	(1,549)	(1,647)
Prior accident years	(3,403)	(3,166)
Total payments	(4,952)	(4,813)
Ending liabilities for unpaid losses and loss adjustment expenses, net	22,814	20,017
Ending liabilities for unpaid losses and loss adjustment expenses, gross	$ 27,897	$ 23,797
Property, Liability and Casualty Insurance Product Line | The Navigators Group, Inc. [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Navigators Group acquisition				$ 2,001	$ 0