Note 3 - Acquisitions (Details Textual) - USD ($) $ in Thousands				3 Months Ended													12 Months Ended
	Oct. 01, 2019	Jan. 08, 2019	May 01, 2017	Dec. 31, 2019	Dec. 31, 2019		Sep. 30, 2019	[2]	Jun. 30, 2019	[2]	Mar. 31, 2019	[3]	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill, Ending Balance				$ 429,597	$ 429,597								$ 172,129				$ 429,597	$ 429,597	$ 429,597	$ 172,129
Revenue from Contract with Customer, Including Assessed Tax					318,751	[1]	$ 284,991		$ 285,650		$ 278,605		269,852	$ 268,268	$ 268,414	$ 265,761		1,167,997		1,072,295	$ 959,956
Net Income (Loss) Attributable to Parent, Total					53,613	[1]	$ 49,835		$ 36,395		$ 38,739		$ 42,008	$ 38,314	$ 43,785	$ 40,653		178,582		164,760	235,171
Amortization of Intangible Assets, Total																		19,200		11,700	8,000
Clearwave [Member]
Revenue from Contract with Customer, Including Assessed Tax																	27,400
Net Income (Loss) Attributable to Parent, Total																	5,100
Amortization of Intangible Assets, Total																	4,900
New Wave [Member]
Payments to Acquire Businesses, Gross			$ 740,200
Goodwill, Ending Balance			$ 87,200	87,200	87,200												87,200	87,200	87,200
Clearwave [Member]
Payments to Acquire Businesses, Gross		$ 358,800
Goodwill, Ending Balance		$ 183,218		185,885	185,885												185,885	185,885	185,885
Goodwill, Purchase Accounting Adjustments																		2,700	2,667
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Income Taxes																		2,700	2,667
Fidelity [Member]
Payments to Acquire Businesses, Gross	$ 531,400
Goodwill, Ending Balance	$ 69,306			71,583	71,583												71,583	71,583	71,583
Goodwill, Purchase Accounting Adjustments				2,277														2,300
Amortization of Intangible Assets, Total					2,400
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	13 years 8 months 12 days
Business Acquisition, Goodwill, Expected Tax Deductible Amount				$ 71,600	71,600												$ 71,600	71,600	$ 71,600
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual					32,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual					$ 4,700
Business Acquisition, Pro Forma Net Income (Loss)																		189,020		159,348
Fidelity [Member] | Adjustments for Depreciation Expense [Member]
Business Acquisition, Pro Forma Net Income (Loss)																		(4,000)		(4,500)
Fidelity [Member] | Adjustments for Amortization Expense [Member]
Business Acquisition, Pro Forma Net Income (Loss)																		6,900		9,200
Fidelity [Member] | Adjustments for Interest Expense [Member]
Business Acquisition, Pro Forma Net Income (Loss)																		10,900		15,200
Fidelity [Member] | Acquisition-related Costs [Member]
Business Acquisition, Pro Forma Net Income (Loss)																		(5,500)		(0)
Fidelity [Member] | Adjustments for Income Tax Provision [Member]
Business Acquisition, Pro Forma Net Income (Loss)																		(2,100)		(5,000)
Selling, General and Administrative Expenses [Member]
Business Combination, Acquisition Related Costs																		$ 9,600		$ 1,800	$ 5,900

[1] Includes Clearwave and Fidelity operations.

[2] Includes Clearwave operations.

[3] Includes Clearwave operations beginning January 8, 2019.