Note 8 - Long-term Debt and Credit Facility (Details Textual) - USD ($)	1 Months Ended	6 Months Ended
	Dec. 31, 2018	Jun. 30, 2019	Jun. 30, 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	$ 314,219,000	$ 307,063,000
Other Loans Payable, Total	1,000,000.0	900,000
Long-term Debt, Fair Value	307,700,000	315,800,000
Derivative, Fixed Interest Rate				2.937%
Interest Rate Swap [Member]
Derivative Asset, Notional Amount		201,796,875
A 2015 Interest Rate Swap [Member]
Debt Instrument, Interest Rate, Effective Percentage					3.68%
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
Long-term Line of Credit, Total	31,000,000.0	38,000,000.0
Letters of Credit Outstanding, Amount		$ 23,600,000
Derivative, Amount of Hedged Item					$ 262,500,000
Credit Facility Maximum Consolidated Leverage Ratio		3.75
Credit Facility Actual Consolidated Leverage Ratio		3.15
Line of Credit Facility, Remaining Borrowing Capacity		$ 60,800,000
Credit Facility Minimum Consolidated Fixed Charge Ratio		1.15
Credit Facility Actual Consolidated Fixed Charge Ratio		1.34
A 2015 Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Interest Rate Swap [Member]
Debt Instrument, Interest Rate, Effective Percentage		4.38%
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	$ 282,200,000	$ 269,100,000
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		7,000,000.0
Letters of Credit Outstanding, Amount		11,300,000
A 2015 Credit Facility [Member] | Insurance Carries Collateral [Member]
Letters of Credit Outstanding, Amount		$ 12,300,000