Note 1 - Summary of Significant Accounting Policies - Calculation of Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2018	[1]	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	[1]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income	$ 16,501		$ 11,614	$ 9,971	$ 4,417	$ 49,298		$ 4,632	$ 1,548	$ (39)	$ 42,503	$ 55,439	$ 16,835
Denominator:
Denominator for basic income per share – weighted-average shares (in shares)											18,340	18,279	18,182
Effect of dilutive securities:
Equivalent shares issuable upon conversion of unvested restricted shares (in shares)											129	93	84
Denominator for diluted income per share adjusted weighted-average shares and assumed conversions (in shares)											18,469	18,372	18,266
Income per share:
Basic income per share (in dollars per share)	$ 0.91		$ 0.63	$ 0.54	$ 0.24	$ 2.70		$ 0.25	$ 0.08	$ 0	$ 2.32	$ 3.03	$ 0.93
Diluted income per share (in dollars per share)	$ 0.89		$ 0.63	$ 0.54	$ 0.24	$ 2.69		$ 0.25	$ 0.08	$ 0	$ 2.30	$ 3.02	$ 0.92

[1] Includes $40.1 million one-time benefit related to the Tax Act.