Note 8 - Long-term Debt and Credit Facility (Details Textual) - USD ($)	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Mar. 12, 2018	Oct. 31, 2015
Line of Credit Facility, Fees		$ 2,500,000
Line of Credit Facility, Up Front Lending Fee		900,000
Line of Credit Facility, Third Party Arrangement Fees		1,600,000
Long-term Debt, Total	$ 302,236,000		$ 314,219,000
Other Loans Payable, Total	700,000		1,000,000.0
Long-term Debt, Fair Value	311,500,000		307,700,000
Derivative, Fixed Interest Rate				2.937%
Interest Rate Swap [Member]
Derivative Asset, Notional Amount	$ 196,875,000
A 2015 Interest Rate Swap [Member]
Derivative Asset, Notional Amount					$ 262,500,000
Derivative, Fixed Interest Rate					1.46%
A 2018 Interest Rate Swap [Member]
Derivative Asset, Notional Amount				$ 170,600,000
Derivative, Fixed Interest Rate				2.937%
Derivative, Amount of Hedged Item				$ 170,600,000
A 2015 Credit Facility [Member]
Debt Instrument, Face Amount					$ 650,000,000.0
Line of Credit Facility, Maximum Borrowing Capacity					350,000,000.0
Line Of Credit Facility, Unamortized Loan Costs		2,400,000
Interest Expense, Debt, Total		$ 200,000
Debt Instrument, Interest Rate, Effective Percentage	3.71%
Long-term Line of Credit, Total	$ 39,000,000.0		31,000,000.0
Letters of Credit Outstanding, Amount	$ 25,200,000
Derivative, Amount of Hedged Item					$ 262,500,000
Credit Facility Maximum Consolidated Leverage Ratio	3.50
Credit Facility Actual Consolidated Leverage Ratio	3.29
Line of Credit Facility, Remaining Borrowing Capacity	$ 19,600,000
Credit Facility Minimum Consolidated Fixed Charge Ratio	1.15
Credit Facility Actual Consolidated Fixed Charge Ratio	1.33
A 2015 Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Interest Rate Swap [Member]
Debt Instrument, Interest Rate, Effective Percentage	4.04%
A 2015 Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Debt Instrument, Basis Spread on Variable Rate	1.25%
A 2015 Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]
Debt Instrument, Basis Spread on Variable Rate	2.25%
A 2015 Credit Facility [Member] | Term Loan [Member]
Debt Instrument, Face Amount	$ 308,400,000
Debt Instrument, Term	5 years
Long-term Debt, Total	$ 262,500,000		$ 282,200,000
A 2015 Credit Facility [Member] | Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 275,000,000.0
Debt Instrument, Term	5 years
A 2015 Credit Facility [Member] | Working Performance Obligation [Member]
Long-term Line of Credit, Total	$ 8,000,000.0
Letters of Credit Outstanding, Amount	12,900,000
A 2015 Credit Facility [Member] | Insurance Carries Collateral [Member]
Letters of Credit Outstanding, Amount	$ 12,300,000