Note 10 - Mergers and Acquisitions (Details)	Jul. 01, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Jun. 29, 2018 USD ($) $ / shares	Mar. 20, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)
Assets		$ 4,106,882,705			$ 3,982,664,773	$ 3,457,186,695
Deposits		3,298,276,027			$ 3,266,655,056
Bates Companies [Member]
Assets under management				$ 700,000,000
Voting interests acquired (as a percent)				100.00%
Cash paid				$ 3,000,000
Issuance of shares of common stock as a result of the acquisition, net of issuance cost (in shares) | shares				21,528
Issuance of shares of common stock as a result of the acquisition, net of issuance cost				$ 1,000,000
Number of shares expecting to issue based on closing stock price | shares				64,583
Springfield Bancshares [Member]
Business acquisition exchange ratio	0.3060
Cash per common share | $ / shares	$ 1.50
Common stock consideration			$ 79,200,000
Share Price | $ / shares			$ 47.45
Total consideration paid			$ 87,400,000
Business Combination, Consideration Transferred			87,400,000
Proceeds from lines of credit		$ 4,900,000	4,100,000
Springfield Bancshares [Member] | SFC Bank [Member]
Assets	$ 573,000,000
Loans with book value	487,000,000
Deposits	$ 439,000,000
Springfield Bancshares [Member] | Revolving Credit Facility [Member]
Line of credit facility, current borrowing capacity			$ 10,000,000
Maximum | Bates Companies [Member]
Common stock consideration				$ 3,000,000