Note 11 - Developments in Accounting Pronouncements (Details Textual) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from Contract with Customer, Including Assessed Tax	$ 143,649	$ 140,839	$ 430,617	$ 417,662
Restricted Cash and Cash Equivalents, Total	1,000	$ 1,100	1,000	1,100		$ 1,000	$ 2,200
Other Nonoperating Income (Expense) [Member]
Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component	$ 700		2,100
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				$ 1,200
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)