Note 11 - Fair Value of Financial Instruments and Fair Value Measurements - Quantitative Information for Financial Instruments Measured at Fair Value (Details) $ in Thousands	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Other Real Estate Owned, fair value	$ 1,635	$ 1,841	$ 4,256
Fair Value, Nonrecurring [Member]
Impaired loans, fair value	1,708	3,352
Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member]
Impaired loans, fair value	1,708	3,352
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member]
Other Real Estate Owned, fair value	$ 1,123	$ 1,183
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Comparability Adjustment [Member]
Other Real Estate Owned, measurement input	(0.3)	(0.3)
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Appraised Value [Member]
Other Real Estate Owned, measurement input	0.0982	0.0982
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Comparability Adjustment [Member]
Other Real Estate Owned, measurement input	0.2502	0.2502
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Appraised Value [Member]
Other Real Estate Owned, measurement input	0.9939	0.9939
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Comparability Adjustment [Member]
Other Real Estate Owned, measurement input	(0.0249)	(0.0249)
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Appraised Value [Member]
Other Real Estate Owned, measurement input	0.3812	0.3526
Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Cap Rate [Member]
Other Real Estate Owned, measurement input		0.1
Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Discount Rate [Member]
Other Real Estate Owned, measurement input	0.1
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member]
Impaired loans, fair value	$ 215	$ 213
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	(0.1086)	(0.1086)
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0	0
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	0.067	0.067
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0.25	0.25
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	(0.0208)	(0.0208)
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0.125	0.125
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member]
Impaired loans, fair value		$ 360
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input		(0.06)
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input		0
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input		19.75
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input		0.1
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input		9.842
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input		0.05
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input		0
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input		0.35
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input		0.175
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member]
Impaired loans, fair value	$ 1,166	$ 2,415
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member]
Impaired loans, measurement input	0
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input		(0.6)
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member]
Impaired loans, measurement input	0.1
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input		0.8
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input		0.1
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0.05
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Cap Rate [Member]
Impaired loans, measurement input	0.105	0.1013
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member]
Impaired loans, fair value	$ 327	$ 328
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	(0.71)	(0.71)
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0.1	0.1
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	(0.035)	(0.035)
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0.8	0.8
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	(0.3725)	(0.3725)
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0.45	0.45
Commercial Portfolio Segment [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member]
Impaired loans, fair value		$ 36
Commercial Portfolio Segment [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input		0
Commercial Portfolio Segment [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Cost to Sell [Member]
Impaired loans, measurement input		0
Commercial Portfolio Segment [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input		0.15
Commercial Portfolio Segment [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Cost to Sell [Member]
Impaired loans, measurement input		0
Commercial Portfolio Segment [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Cap Rate [Member]
Impaired loans, measurement input		0.15
Commercial Portfolio Segment [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Cost to Sell [Member]
Impaired loans, measurement input		0