Business Acquisitions - Additional Information (Details) - The Navigators Group, Inc. [Member] - USD ($) $ in Millions		3 Months Ended
May 23, 2019	Mar. 31, 2020
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	May 23, 2019
Payments to Acquire Businesses, Gross	$ 2,121
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments Related to Previous Period		$ 0

Date when the acquirer obtains control of the acquiree, in CCYY-MM-DD format.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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Amount of increase (decrease) in income that would have been recognized in previous periods if the adjustments to provisional amounts were recognized as of the acquisition date.

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The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

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