Note 11 - Deferred Compensation Plan - Summary of Deferred Compensation Plan (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Deferred compensation plan assets	$ 30,425	$ 28,080
Deferred compensation plan liabilities	30,859	28,443
Other Long-term Assets [Member]
Deferred compensation plan assets	30,425	28,080
Accrued Compensation and Related Benefits [Member]
Deferred compensation plan liabilities	447	356
Other Long-term Liabilities [Member]
Deferred compensation plan liabilities	30,412	28,087
Cash Surrender Value [Member]
Deferred compensation plan assets	14,141	11,455
Mutual Funds and Money Market Funds, Fair Value [Member]
Deferred compensation plan assets	$ 16,284	$ 16,625