Note 9 - Debt, Credit Facilities and Leases (Details Textual) $ in Millions				3 Months Ended		12 Months Ended
		May 01, 2019	Apr. 12, 2013 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2014 USD ($)	Jan. 01, 2020	Oct. 01, 2019	Apr. 01, 2019	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 31, 2018 USD ($)	Mar. 05, 2018 USD ($)	Mar. 05, 2018 CAD ($)	May 31, 2016 USD ($)
Letters of Credit Outstanding, Amount				$ 3,000,000
Finance Lease, Liability, Total				15,160,000							$ 13,100,000
Finance Lease, Liability, Current				5,858,000							5,264,000
Finance Lease, Liability, Noncurrent				9,302,000							7,871,000
Finance Lease, Right-of-Use Asset				21,900,000							20,000,000
Finance Lease, Right-of-Use Asset, Amortization				1,600,000
Finance Lease, Interest Expense				200,000
Finance Lease, Liability, Payment, Due, Total				16,185,000
Finance Lease, Liability, Undiscounted Excess Amount				1,025,000
Operating Lease, Liability, Total				20,665,000						$ 22,400,000
Operating Lease, Liability, Current				6,701,000
Operating Lease, Liability, Noncurrent				13,964,000
Operating Lease, Right-of-Use Asset				20,647,000						$ 22,400,000
Operating Lease, Expense				2,100,000
Lessee, Operating Lease, Liability, Payments, Due, Total				$ 22,899,000
Minimum [Member]
Lessee, Operating Lease, Discount Rate				5.00%
Maximum [Member]
Leverage Ratio	[1]			4.5
Lessee, Operating Lease, Discount Rate				6.50%
Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity												$ 250,000,000			$ 100,000,000
Long-term Line of Credit, Total				$ 0
Repayments of Lines of Credit				58,000,000
Proceeds from Lines of Credit, Total				85,000,000
Revolving Credit Facility [Member] | Forecast [Member]
Leverage Ratio							4	4.5
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												3.25%
Subsequent Event [Member] | Revolving Credit Facility [Member]
Leverage Ratio									5
Senior Notes [Member]
Debt Instrument, Face Amount			$ 500,000,000
Payment for Pension Benefits						$ 6,500,000
Proceeds from Issuance of Senior Long-term Debt			$ 490,000,000
Underwriting Discount on Senior Notes			2.00%
Debt Instrument, Unamortized Discount, Total			$ 10,000,000	2,700,000
Debt Instrument, Interest Rate, Stated Percentage			6.875%
Interest Expense, Debt, Total				9,100,000	$ 9,100,000
Debt Instrument, Redemption Price, Percentage			101.00%
Senior Notes [Member] | Subsequent Event [Member]
Debt Instrument, Redemption Price, Percentage		100.00%
Series 2018-A Senior Notes [Member]
Debt Instrument, Face Amount													$ 30,800,000	$ 40.0
Debt Instrument, Unamortized Discount, Total														$ 0.2
Debt Instrument, Interest Rate, Stated Percentage													4.68%	4.68%
Interest Expense, Debt, Total				$ 400,000	$ 400,000
Debt Instrument, Issuance, Discount Rate													0.58%	0.58%

[1] The leverage ratio changed to 5.00:1 effective April 1, 2019, will return to 4.50:1 effective October 1, 2019, and then change to 4.00:1 effective January 1, 2020.