Business Acquisitions - Fair Value of Assets Acquired and Liabilities Assumed at the Acquisition Date (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2019	May 23, 2019
Business Acquisition [Line Items]
Goodwill		$ 1,911	$ 1,913
The Navigators Group, Inc. [Member]
Business Acquisition [Line Items]
Cash and invested assets	$ 3,851			$ 3,848
Premiums receivable	498			492
Reinsurance recoverables	1,097			1,100
Prepaid reinsurance premiums	238			238
Other intangible assets	580			580
Property and equipment	83			83
Other assets	102			99
Total assets acquired	6,449			6,440
Unpaid losses and loss adjustment expenses	2,823			2,823
Unearned premiums	1,219			1,219
Long-term debt	284			284
Deferred income taxes, net	47			48
Other liabilities	576			568
Total liabilities assumed	4,949			4,942
Net identifiable assets acquired	1,500			1,498
Goodwill	621			623
Net assets acquired	2,121			$ 2,121
Cash and invested assets, adjustment	3
Premiums receivable, adjustment	6
Reinsurance recoverables, adjustment	(3)
Prepaid reinsurance premiums, adjustment	0
Other intangible assets, adjustment	0
Property and equipment, adjustment	0
Other assets, adjustment	3
Total assets acquired, adjustment	9
Unpaid losses and loss adjustment expenses, adjustment	0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Revenue	0
Long-term debt, adjustment	0
Deferred income taxes, net, adjustment	(1)
Other liabilities, adjustment	8
Total liabilities assumed, adjustment	7
Net identifiable assets acquired, adjustment	2
Goodwill, adjustment	2
Net assets acquired, adjustment	$ 0