Note 10 - Developments in Accounting Pronouncements (Details Textual) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from Contract with Customer, Including Assessed Tax	$ 147,259		$ 134,279	$ 286,968	$ 276,823
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			1,400
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					$ 1,100
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)