Note 9 - Long-term Debt and Credit Facility (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Mar. 12, 2018	Oct. 31, 2015
Long-term Debt, Total	$ 222,467	$ 278,520	$ 278,520
Payments of Financing Costs, Total	1,995		(0)	$ 1,657
Repayments of Lines of Credit		700
Long-term Debt, Fair Value	219,200	286,800	286,800
Derivative, Fixed Interest Rate					2.937%
A 2015 Interest Rate Swap [Member]
Derivative Asset, Notional Amount						$ 262,500
Derivative, Fixed Interest Rate						1.46%
A 2018 Interest Rate Swap [Member]
Derivative Asset, Notional Amount					$ 170,600
Derivative, Fixed Interest Rate					2.937%
Derivative, Amount of Hedged Item					$ 170,600
Term Loan [Member]
Long-term Debt, Total	$ 221,700	253,800	253,800
The 2015 Credit Facility [Member]
Debt Instrument, Face Amount						$ 650,000
Line of Credit Facility, Maximum Borrowing Capacity						350,000
Debt Instrument, LIBOR Floor	0.25%
Other Loans Payable, Total	$ 700	800	800
Letters of Credit Outstanding, Amount	$ 30,800
Long-term Line of Credit, Total		$ 24,000	$ 24,000
Derivative, Amount of Hedged Item						$ 262,500
Credit Facility Maximum Consolidated Leverage Ratio	3.50
Credit Facility Actual Consolidated Leverage Ratio	2.32
Line of Credit Facility, Remaining Borrowing Capacity	$ 118,200
Credit Facility Minimum Consolidated Fixed Charge Ratio	1.15
Credit Facility Actual Consolidated Fixed Charge Ratio	1.40
The 2015 Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate	2.00%
The 2015 Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Debt Instrument, Basis Spread on Variable Rate	1.25%
The 2015 Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]
Debt Instrument, Basis Spread on Variable Rate	3.25%
The 2015 Credit Facility [Member] | Term Loan [Member]
Long-term Debt, Total	$ 308,400
Debt Instrument, Interest Rate, Effective Percentage	4.09%
The 2015 Credit Facility [Member] | Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 175,000
Payments of Financing Costs, Total	2,200			3,100
Line Of Credit Facility, Unamortized Loan Costs	1,900			2,400
Interest Expense, Debt, Total	200			600
Repayments of Long-term Lines of Credit	24,000
The 2015 Credit Facility [Member] | Revolving Credit Facility [Member] | Up-front Lending Fees [Member]
Payments of Financing Costs, Total	1,700			1,400
The 2015 Credit Facility [Member] | Revolving Credit Facility [Member] | Third-party Arranging Fees and Expenses [Member]
Payments of Financing Costs, Total	500			$ 1,700
The 2015 Credit Facility [Member] | Insurance Carries Collateral [Member]
Letters of Credit Outstanding, Amount	11,200
The 2015 Credit Facility [Member] | Working Performance Obligation [Member]
Letters of Credit Outstanding, Amount	$ 19,600