Business Acquisitions - Additional Information (Details) - The Navigators Group, Inc. [Member] - USD ($) $ in Millions		3 Months Ended
	May 23, 2019	Jun. 30, 2020
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	May 23, 2019
Payments to Acquire Businesses, Gross	$ 2,121
Total assets acquired, adjustment		$ 9
Total liabilities assumed, adjustment		7
Goodwill, adjustment		$ 2