Note 17 - Stock-based Compensation (Details Textual) - USD ($)		3 Months Ended		12 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized					500,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant				547,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance				0	0
Proceeds (Used for) and Received from Settlement of Stock Awards				$ 272,000	$ 154,000
Share-based Payment Arrangement, Expense, Tax Benefit				$ 751,000	846,000	$ 607,000
Common Stock, Capital Shares Reserved for Future Issuance				118,000
Share-based Compensation Arrangement by Share-based Payment Award, Expected Shares to be Awarded				156,000
Accounting Standards Update 2016-09 [Member]
Share-based Payment Arrangement, Expense, Tax Benefit				$ 812,000	$ 549,000	$ 1,500,000
Stock Appreciation Rights (SARs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	[1]			547,000
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period				10 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period				53,000	100,000	46,000
Share-based Payment Arrangement, Expense, Tax Benefit				$ 72,000	$ 80,000	$ 59,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Exercise Price				$ 25.06
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number, Ending Balance				641,000
Stock Appreciation Rights (SARs) [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Exercise Price				$ 14.02	$ 14.02	$ 14.02	$ 14.02
Stock Appreciation Rights (SARs) [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Exercise Price				$ 40.00	$ 40.00	$ 40.00	$ 33.08
Stock Appreciation Rights (SARs) [Member] | Share-based Payment Arrangement, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage				20.00%
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period				5 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period				40,000	40,000	29,000
Share-based Payment Arrangement, Expense, Tax Benefit				$ 249,000	$ 231,000	$ 216,000
Common Stock, Capital Shares Reserved for Future Issuance				108,000
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period				3 years
Sharebased Compensation Arrangement by Share-based Payment Award Post Vesting Holding Period				1 year
Share-based Compensation Arrangement by Share-based Payment Award Liquidity Discount				4.10%	4.30%	5.10%
Share-based Payment Arrangement, Expense, Tax Benefit				$ 361,000	$ 483,000	$ 286,000
Performance Shares [Member] | Minimum [Member]
Common Stock, Capital Shares Reserved for Future Issuance				0
Performance Shares [Member] | Maximum [Member]
Common Stock, Capital Shares Reserved for Future Issuance				196,000
Restricted Stock Units (RSUs) [Member]
Share-based Payment Arrangement, Expense, Tax Benefit				$ 69,000	$ 52,000	$ 46,000
Common Stock, Capital Shares Reserved for Future Issuance				10,000
Restricted Stock Units (RSUs) [Member] | Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period				12 months
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period		9,834	6,525
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Fair Value Granted		$ 330,000	$ 247,000

[1] Under the 2015 Omnibus Equity Compensation Plan, shares of stock are authorized for issuance as incentive and non-qualified stock options, SARs, RSAs, and RSUs.