Note 12 - Derivatives - Summary of Gains (Losses) Recognized in Statement of Operations and AOCI (Details) - Cash Flow Hedging [Member] - Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Derivative gain (loss) reclassified from accumulated OCI to current earnings	$ 250	$ 711
Other Comprehensive Income (Loss) [Member]
Derivative gain (loss) recognized into OCI	(11,130)	(3,242)
Other Comprehensive Income (Loss) [Member] | Natural Gas Contracts [Member]
Derivative gain (loss) recognized into OCI	(1,755)	1,194
Other Comprehensive Income (Loss) [Member] | Interest Rate Swap [Member]
Derivative gain (loss) recognized into OCI	(9,375)	(4,436)
Cost of Sales [Member] | Natural Gas Contracts [Member]
Derivative gain (loss) reclassified from accumulated OCI to current earnings	(651)	426
Interest Expense [Member] | Interest Rate Swap [Member]
Derivative gain (loss) reclassified from accumulated OCI to current earnings	$ 901	$ 285