Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Liabilities for Unpaid Losses and Loss Adjustment Expenses (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 23, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	$ 33,029,000,000
Provision for unpaid losses and loss adjustment expenses
Change in Deferred Gain on Retroactive Reinsurance	(16,000,000)	$ 0	$ 0
Payments
Ending liabilities for unpaid losses and loss adjustment expenses, gross	36,517,000,000	33,029,000,000
Property and Casualty Insurance Products
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	24,584,000,000	23,775,000,000	22,545,000,000
Reinsurance and other recoverables	4,232,000,000	3,957,000,000	3,488,000,000
Beginning liabilities for unpaid losses and loss adjustment expenses, net	20,352,000,000	19,818,000,000	19,057,000,000
Provision for unpaid losses and loss adjustment expenses
Total provision for unpaid losses and loss adjustment expenses	7,398,000,000	6,940,000,000	7,340,000,000
Payments
Current accident year	(2,374,000,000)	(2,452,000,000)	(2,751,000,000)
Prior accident years	(4,374,000,000)	(3,954,000,000)	(3,828,000,000)
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss)	(1,000,000)	0	0
Ending liabilities for unpaid losses and loss adjustment expenses, net	22,986,000,000	20,352,000,000	19,818,000,000
Reinsurance and other recoverables	5,275,000,000	4,232,000,000	3,957,000,000
Ending liabilities for unpaid losses and loss adjustment expenses, gross	$ 28,261,000,000	24,584,000,000	23,775,000,000
Property and Casualty Insurance Products | The Navigators Group, Inc. [Member]
Product Information [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Reserves net Reinsurance and Other Recoverables		$ 0	$ 0	$ 2,001,000,000