Note 10 - Developments in Accounting Pronouncements (Details Textual) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from Contract with Customer, Including Assessed Tax	$ 139,709	$ 142,544
Restricted Cash and Cash Equivalents, Total	1,000	2,200		$ 1,000	$ 2,200
Cost of Goods Sold Before Depreciation, Depletion, and Amortization	72,869	78,676
Other Nonoperating Income (Expense) [Member]
Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component	700
Scenario, Forecast [Member] | Other Nonoperating Income (Expense) [Member]
Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component			$ 2,800
Accounting Standards Update 2014-09 [Member] | Pro Forma [Member]
Revenue from Contract with Customer, Including Assessed Tax		140
Cost of Goods Sold Before Depreciation, Depletion, and Amortization		$ 140
Accounting Standards Update 2016-01 [Member] | AOCI Attributable to Parent [Member]
Reclassification from Accumulated Other Comprehensive Income to Retained Earnings	1,300
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]
Reclassification from Accumulated Other Comprehensive Income to Retained Earnings	$ (1,300)