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Note 2 - Acquisitions and Divestitures - Schedule of Business Acquisition Assets and Liabilities of Valley Forge Convention Center Partners (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 17, 2018	Dec. 31, 2017
Goodwill	$ 971,287	$ 1,083,287	$ 1,062,102		$ 888,224
Valley Forge Convention Center Partners, L.P. [Member]
Property and equipment	57,118
Intangible assets	136,600
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net, Total				$ 289,100
Revision of Prior Period, Adjustment [Member] \| Valley Forge Convention Center Partners, L.P. [Member]
Current assets	29,280
Property and equipment	57,118
Other assets	2,872
Intangible assets	136,600
Total acquired assets	225,870
Current liabilities	12,968
Other liabilities	9,803
Total liabilities assumed	22,771
Net identifiable assets acquired	203,099
Goodwill	85,966
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net, Total	$ 289,065