Note 9 - Acquisitions (Details Textual)	Apr. 18, 2018 USD ($) $ / shares	Mar. 20, 2018 USD ($) shares	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)
Assets, Total			$ 4,026,314,414	$ 3,982,664,773	$ 3,381,012,618
Deposits, Total			3,280,001,492	$ 3,266,655,056
Springfield Bancshares [Member]
Assets, Total			563,200,000
Deposits, Total			$ 446,500,000
Bates Companies [Member]
Assets under Management, Carrying Amount		$ 700,000,000
Business Acquisition, Percentage of Voting Interests Acquired		100.00%
Payments to Acquire Businesses, Gross		$ 3,000,000
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable		$ 3,000,000
Stock Issued During Period, Shares, Acquisitions | shares		21,528
Stock Issued During Period, Value, Acquisitions		$ 1,000,000
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares		64,583
Springfield Bancshares [Member] | Subsequent Event [Member]
Business Acquisition, Percentage of Voting Interests Acquired	100.00%
Business Acquisition, Shares Exchanged, Per Share Amount	0.306
Business Acquisition, Shares Exchanged, Per Share Value | $ / shares	$ 1.50
Business Combination, Consideration Transferred, Total	$ 86,700,000