Note 7 - Commitments and Contingencies (Details Textual) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Capital Addition Purchase Commitments [Member]
Purchase Obligation, Future Minimum Payments, Remainder of Fiscal Year	$ 140,900,000
Purchase Obligation, Due in Second Year	39,400,000
Line of Credit [Member]
Letters of Credit Outstanding, Amount	$ 37,500,000	$ 34,500,000

Minimum amount of purchase arrangement maturing in the remainder of the fiscal year following the latest fiscal year ended. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

No definition available.

Name:	us-gaap_PurchaseObligationFutureMinimumPaymentsRemainderOfFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant