Note 7 - Commitments and Contingencies (Details Textual) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Letters of Credit Outstanding, Amount	$ 34,800	$ 36,300

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

No definition available.

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