Note 6 - Debt, Credit Facility and Leases (Details Textual) $ in Thousands, shares in Millions, $ in Millions			1 Months Ended		12 Months Ended
	May 01, 2019	Apr. 12, 2013 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2019 CAD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2020	Jul. 01, 2020	Jun. 30, 2020	Mar. 31, 2020	Feb. 06, 2020 USD ($)	Jan. 01, 2020	Sep. 30, 2019	Jul. 31, 2019 USD ($)	Jul. 31, 2018 USD ($)	Mar. 05, 2018 USD ($)	Mar. 05, 2018 CAD ($)	May 31, 2016 USD ($)
Paid-in-Kind Prepayment of Debt Fee					$ 2,855	$ 0	$ 0
Finance Lease, Liability, Total			$ 12,643		12,643	13,100
Finance Lease, Liability, Current			5,429		5,429	5,264
Finance Lease, Liability, Noncurrent			7,214		7,214	7,871
Finance Lease, Right-of-Use Asset			20,600		20,600	20,000
Finance Lease, Right-of-Use Asset, Amortization					5,900	5,300	7,100
Finance Lease, Interest Expense					700	700	600
Finance Lease, Liability, Payment, Due, Total			13,449		13,449
Finance Lease, Liability, Undiscounted Excess Amount			$ 806		$ 806
Finance Lease, Weighted Average Remaining Lease Term			1 year 9 months 18 days		1 year 9 months 18 days
Finance Lease, Weighted Average Discount Rate, Percent			6.40%		6.40%
Operating Lease, Liability, Total			$ 16,398		$ 16,398
Operating Lease, Liability, Current			5,580		5,580	0
Operating Lease, Liability, Noncurrent			10,818		10,818	0
Operating Lease, Right-of-Use Asset			16,381		16,381	0
Operating Lease, Expense					7,500
Lessee, Operating Lease, Liability, Payments, Due, Total			$ 18,812		$ 18,812
Operating Lease, Weighted Average Remaining Lease Term			4 years 3 months 18 days		4 years 3 months 18 days
Minimum [Member]
Lessee, Operating Lease, Discount Rate			5.00%		5.00%
Maximum [Member]
Lessee, Operating Lease, Discount Rate			6.50%		6.50%
Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity																$ 150,000	$ 250,000			$ 100,000
Leverage Ratio			6		6										4
Letters of Credit Outstanding, Amount			$ 33,600		$ 33,600
Long-term Line of Credit, Total			$ 0		0
Revolving Credit Facility [Member] | Subsequent Event [Member]
Line of Credit Facility, Amount Available for Refinance													$ 100,000
Line of Credit Facility, Amount Available for Refinance, Conditions, New Debt Required to Be Issued													400,000
Amount of New Debt Raised After Which Dollar for Dollar Reduction in Line of Credit Will Take Effect													600,000
Revolving Credit Facility [Member] | Forecast [Member]
Line of Credit Facility, Maximum Borrowing Capacity													$ 250,000
Leverage Ratio									4.00	4.00	4.25	4.25		4.25
Letter of Credit [Member]
Letter of Credit Outstanding, Fronting Fee																	0.20%
Letter of Credit [Member] | Minimum [Member]
Letter of Credit, Participation Fee, Percent																	2.25%
Letter of Credit [Member] | Maximum [Member]
Letter of Credit, Participation Fee, Percent																	4.00%
Conversion of Series 2018A Senior Notes Into Common Stock [Member]
Debt Conversion, Converted Instrument, Shares Issued | shares			10.7	10.7
Debt Conversion, Converted Instrument, Amount			$ 33,500	$ 43.8
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	1,800		1,800
Debt Instrument, Interest Rate, Stated Percentage		6.875%
Interest Expense, Debt, Total					36,300	$ 36,300	35,300
Interest Costs Capitalized							900
Long-term Debt, Total			504,700		504,700
Long-term Debt, Gross			506,500		506,500
Debt Instrument, Unamortized Discount (Premium), Net, Total			1,800		1,800
Interest Payable			$ 5,800		5,800
Debt Instrument, Redemption Price, Percentage	100.00%					101.00%
Senior Notes [Member] | Costs Related to Private Offering and Tender Offer [Member]
Interest Expense, Debt, Total							$ 900
Series 2018-A Senior Notes [Member]
Debt Instrument, Face Amount																		$ 30,800	$ 40.0
Debt Instrument, Unamortized Discount, Total																			$ 0.2
Debt Instrument, Interest Rate, Stated Percentage																		4.68%	4.68%
Interest Expense, Debt, Total					4,200	$ 1,400
Debt Instrument, Issuance, Discount Rate																		0.58%	0.58%
Paid-in-Kind Prepayment of Debt Fee					$ 2,900