Note 9 - Debt, Credit Facilities and Leases (Details Textual) $ in Thousands, $ in Millions				1 Months Ended	6 Months Ended	12 Months Ended
		May 01, 2019	Apr. 12, 2013 USD ($)	Aug. 07, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 31, 2018 USD ($)	Mar. 05, 2018 CAD ($)	Mar. 05, 2018 USD ($)	May 31, 2016 USD ($)
Leverage Ratio	[1]				4.50
Letters of Credit Outstanding, Amount					$ 3,000
Finance Lease, Liability, Total					13,405								$ 13,100
Finance Lease, Liability, Current					5,392								5,264
Finance Lease, Liability, Noncurrent					8,013								7,871
Finance Lease, Right-of-Use Asset					20,100								20,000
Finance Lease, Right-of-Use Asset, Amortization					3,400
Finance Lease, Interest Expense					400
Finance Lease, Liability, Payment, Due, Total					14,265
Finance Lease, Liability, Undiscounted Excess Amount					$ 860
Finance Lease, Weighted Average Remaining Lease Term					1 year 9 months 18 days
Operating Lease, Liability, Total					$ 19,038							$ 22,400
Operating Lease, Liability, Current					6,628								0
Operating Lease, Liability, Noncurrent					12,410								0
Operating Lease, Right-of-Use Asset					19,019								$ 0
Operating Lease, Expense					4,000
Lessee, Operating Lease, Liability, Payments, Due, Total					$ 19,170
Operating Lease, Weighted Average Remaining Lease Term					4 years 6 months
Subsequent Event [Member]
Letters of Credit Outstanding, Amount				$ 23,000
Maximum [Member]
Lessee, Operating Lease, Discount Rate					6.50%
Minimum [Member]
Lessee, Operating Lease, Discount Rate					5.00%
Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity					$ 150,000									$ 250,000			$ 100,000
Proceeds from Lines of Credit, Total					52,000
Revolving Credit Facility [Member] | Forecast [Member]
Leverage Ratio							4.00	5.00	5.50	6.00	6.50
Revolving Credit Facility [Member] | Scenario, Election One [Member]
Line of Credit Facility, Maximum Borrowing Capacity					250,000
Revolving Credit Facility [Member] | Scenario, Election Two [Member]
Line of Credit Facility, Maximum Borrowing Capacity					$ 250,000
Revolving Credit Facility [Member] | Scenario, Election Two [Member] | Maximum [Member]
Leverage Ratio					4.00
Letter of Credit [Member]
Letter of Credit Outstanding, Fronting Fee														0.20%
Letter of Credit [Member] | Subsequent Event [Member]
Proceeds from Long-term Lines of Credit				$ 20,000
Letter of Credit [Member] | Maximum [Member]
Letter of Credit, Participation Fee, Percent														3.25%
Letter of Credit [Member] | Minimum [Member]
Letter of Credit, Participation Fee, Percent														2.25%
Senior Notes [Member]
Debt Instrument, Face Amount			$ 500,000
Payment for Pension Benefits						$ 6,500
Proceeds from Issuance of Senior Long-term Debt			$ 490,000
Underwriting Discount on Senior Notes			2.00%
Debt Instrument, Unamortized Discount, Total			$ 10,000		$ 2,400
Debt Instrument, Interest Rate, Stated Percentage			6.875%
Interest Expense, Debt, Total					18,100
Debt Instrument, Redemption Price, Percentage		100.00%	101.00%
Series 2018-A Senior Notes [Member]
Debt Instrument, Face Amount															$ 40.0	$ 30,800
Debt Instrument, Unamortized Discount, Total															$ 0.2
Debt Instrument, Interest Rate, Stated Percentage															4.68%	4.68%
Interest Expense, Debt, Total					$ 700
Debt Instrument, Issuance, Discount Rate															0.58%	0.58%

[1] EBITDA is calculated as defined in the credit agreement.