Note 9 - Stockholders' Equity (Details Textual) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Feb. 14, 2019	Dec. 31, 2018	Jul. 25, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 25, 2017
Common Stock, Shares Authorized		37,000,000		37,000,000
Preferred Stock, Shares Authorized		3,000,000		3,000,000
Preferred Stock, Shares Outstanding, Ending Balance		0		0
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized		5,500,000
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period		10 years
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		7,118,003
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant		1,212,644
Share Price		$ 23.64		$ 31.84
Allocated Share-based Compensation Expense, Total		$ 2,349		$ 2,256	$ 1,680
Common Stock, Dividends, Per Share, Declared		$ 0.28		$ 0.22	$ 0.18
Subsequent Event [Member]
Common Stock, Dividends, Per Share, Declared	$ 0.085
Dividends Payable, Date to be Paid	Apr. 05, 2019
Dividends Payable, Date of Record	Mar. 22, 2019
Dividends Payable	$ 1,700
Stock Repurchase Program [Member]
Stock Repurchased During Period, Shares			0
Stock Repurchase Program [Member] | Common Stock [Member]
Stock Repurchase Program, Authorized Amount						$ 7,500
Stock Repurchase Program [Member] | Common Stock [Member] | Subsequent Event [Member]
Stock Repurchase Program, Authorized Amount	$ 10,000
General and Administrative Expense [Member]
Allocated Share-based Compensation Expense, Total		$ 1,293		$ 1,484	$ 983
General and Administrative Expense [Member] | President [Member]
Allocated Share-based Compensation Expense, Total				500
Employee Stock Option [Member]
Allocated Share-based Compensation Expense, Total		$ 1,457		1,612	1,116
Employee Stock Option [Member] | Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		4 years
Employee Stock Option [Member] | Share-based Compensation Award, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		5 years
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value		$ 1,900		1,900	800
Allocated Share-based Compensation Expense, Total		892		$ 644	$ 564
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total		$ 9,700
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition		3 years 292 days
Stock Repurchased During Period, Shares		21,410		27,291