Note 2 - Mergers and Acquisitions (Details Textual)			3 Months Ended	12 Months Ended
	Jan. 01, 2019 USD ($)	Jan. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)
Goodwill, Ending Balance			$ 15,836,000	$ 15,836,000	$ 12,124,000
Finite-Lived Intangible Assets, Net, Ending Balance			2,786,000	2,786,000	1,498,000
Business Combination, Acquisition Related Costs				$ 2,198,000	$ 1,169,000
Effective Income Tax Rate Reconciliation, Percent, Total				22.16%	15.50%
Core Deposits [Member]
Finite-Lived Intangible Assets, Net, Ending Balance			2,786,000	$ 2,786,000	$ 1,498,000
Finite-Lived Intangible Asset, Useful Life				10 years
Big Muddy Bancorp Acquisition [Member]
Business Combination, Consideration Transferred, Total	$ 16,436,000
Payments to Acquire Businesses, Gross	1,000
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable	$ 16,435,000
Business Acquisition, Percentage of Voting Interests Acquired	100.00%
Goodwill, Ending Balance	$ 3,586,000		3,712,000	$ 3,712,000
Goodwill, Purchase Accounting Adjustments			126,000	126,000
Business Combination, Acquired Loans Discount	2,813,000		1,333,000	1,333,000
Business Combination, Accretion of Loan Discount				1,480,000
Business Acquisition, Impairment Loans	$ 556,000			175,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, and Equipment				276,000
Business Combination, Acquisition Related Costs				$ 1,380,000	804,000
Effective Income Tax Rate Reconciliation, Percent, Total				20.00%
Big Muddy Bancorp Acquisition [Member] | Core Deposits [Member]
Finite-Lived Intangible Assets, Net, Ending Balance			1,988,000	$ 1,988,000
TwinCo Acquisition [Member]
Business Combination, Consideration Transferred, Total		$ 18,930,000
Payments to Acquire Businesses, Gross		9,900,000
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable		$ 9,030,000
Business Acquisition, Percentage of Voting Interests Acquired		100.00%
Goodwill, Ending Balance		$ 5,090,000				$ 5,090,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Investments		(941,000)
Business Combination, Acquired Loans Discount		1,834,000	836,000	836,000
Business Combination, Accretion of Loan Discount				409,000	589,000
Business Acquisition, Impairment Loans		1,188,000		1,061,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, and Equipment				446,000
Business Combination, Acquisition Related Costs		$ 1,041,000			$ 365,000
TwinCo Acquisition [Member] | Core Deposits [Member]
Finite-Lived Intangible Assets, Net, Ending Balance			$ 1,609,000	$ 1,609,000
SBOT [Member] | Townsend, Dutton, Denton and Choteau, Montana [Member]
Number of Bank Branches	4
Ruby Valley Bank [Member] | Madison County, Montana [Member]
Number of Bank Branches		2