Note 11 - Deferred Compensation Plan - Summary of Deferred Compensation Plan (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Deferred compensation plan assets	$ 29,224	$ 28,080
Deferred compensation plan liabilities	29,644	28,443
Other Long-term Assets [Member]
Deferred compensation plan assets	29,224	28,080
Accrued Compensation and Related Benefits [Member]
Deferred compensation plan liabilities	447	356
Other Long-term Liabilities [Member]
Deferred compensation plan liabilities	29,197	28,087
Cash Surrender Value [Member]
Deferred compensation plan assets	12,111	11,455
Mutual Funds and Money Market Funds, Fair Value [Member]
Deferred compensation plan assets	$ 17,113	$ 16,625