Note 11 - Fair Value of Financial Instruments and Fair Value Measurements - Quantitative Information for Financial Instruments Measured at Fair Value (Details) $ in Thousands	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Other Real Estate Owned, fair value	$ 2,173	$ 4,256	$ 6,439
Fair Value, Measurements, Nonrecurring [Member]
Impaired loans, fair value	5,185	4,876
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Impaired loans, fair value	5,185	4,876
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Other Real Estate Owned, fair value	$ 1,468	$ 2,015
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Comparability Adjustment [Member]
Other Real Estate Owned, measurement input	(0.476)	(0.2274)
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Appraised Value [Member]
Other Real Estate Owned, measurement input	0.0982	0.0544
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Comparability Adjustment [Member]
Other Real Estate Owned, measurement input	0.2502	0.15
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Appraised Value [Member]
Other Real Estate Owned, measurement input	0.8121	0.8724
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Comparability Adjustment [Member]
Other Real Estate Owned, measurement input	(0.1129)	(0.0387)
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Appraised Value [Member]
Other Real Estate Owned, measurement input	0.3448	0.2444
Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, 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, Measurements, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Discount Rate [Member]
Other Real Estate Owned, measurement input	0.1
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Impaired loans, fair value	$ 427	$ 427
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	(0.16)	(0.16)
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0	0
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	19.75	19.75
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0.1	0.1
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	9.795	9.795
Real Estate Portfolio Segment [Member] | Commercial Construction [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0.05	0.05
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Impaired loans, fair value	$ 15	$ 82
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	(0.433)	(0.433)
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0.1	0
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	0.667	0.833
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, 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, Measurements, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	0.117	0.2
Real Estate Portfolio Segment [Member] | Residential [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0.175	0.125
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, 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, Measurements, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Cost to Sell [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, Measurements, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	10
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Cost to Sell [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, Measurements, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	0.0474
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Impaired loans, fair value	$ 4,406	$ 4,017
Real Estate Portfolio Segment [Member] | Commercial [Member] | Valuation, Income Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Minimum [Member] | Measurement Input, Appraised Value [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, Measurements, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Appraised Value [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, Measurements, 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, Measurements, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Cap Rate [Member]
Impaired loans, measurement input	0.1075	0.1075
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Impaired loans, fair value	$ 337	$ 350
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, 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, Measurements, 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, Measurements, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input	0.887	0.887
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Maximum [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	80	0.75
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Comparability Adjustment [Member]
Impaired loans, measurement input		0.0885
Real Estate Portfolio Segment [Member] | Farmland [Member] | Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Weighted Average [Member] | Measurement Input, Appraised Value [Member]
Impaired loans, measurement input	0.0885	0.425
Other Real Estate Owned, measurement input	0.45