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Acquisitions Of Businesses (Details) $ in Thousands										9 Months Ended	12 Months Ended
	Aug. 03, 2020 Company	Jan. 02, 2020 Company	Nov. 01, 2019 USD ($) Company	Oct. 31, 2019 Company	Sep. 16, 2019 Company	Aug. 01, 2019 Company	May 10, 2019 Company	Apr. 01, 2019 Company	Jan. 04, 2019 Company	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Goodwill										$ 846,908	$ 1,063,911
Number of businesses acquired | Company	1	1	1	1	1	1	1	1	1
BKI [Member]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired			100.00%
Purchase price			$ 220,300
Working capital acquired			29,800
Other net assets			4,900
Goodwill			43,900
Identifiable intangible assets			$ 141,700
Amortization periods			10 years 6 months
2019 Acquisitions, excluding BKI
Business Acquisition [Line Items]
Purchase price											85,400
Working capital acquired											25,300
Other net assets											1,300
Goodwill											29,200
Identifiable intangible assets											$ 29,600
2020 Acquisitions
Business Acquisition [Line Items]
Purchase price										43,200
Working capital acquired										2,400
Other net liabilities										4,000
Goodwill										8,100
Identifiable intangible assets										$ 36,700

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Percentage of voting equity interests acquired at the acquisition date in the business combination.

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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