Note 12 - Regulatory Matters (Details Textual) - USD ($) $ / shares in Units, $ in Millions									3 Months Ended		9 Months Ended
	Dec. 15, 2021	Oct. 14, 2021	Sep. 15, 2021	Jul. 15, 2021	Jun. 16, 2021	Apr. 15, 2021	Mar. 17, 2021	Jan. 14, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	May 31, 2021	Dec. 31, 2020	May 31, 2019	Dec. 31, 2009
Preferred Securities of Subsidiary Trust									$ 46.0		$ 46.0			$ 45.5
Maximum Restricted Core Element Allowed in Tier One Capital Percent											25.00%
Maximum Level of Consolidated Aggregate Assets Allowing for Inclusion of Trust Preferred Securities in Tier One Capital																$ 15,000.0
Trust Preferred Securities Included in Tier One Capital									$ 46.0		$ 46.0			$ 45.5
Capital Conservation Buffer									2.50%		2.50%
Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy to Risk Weighted Assets									0.070		0.070
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets									0.085		0.085
Capital Required for Capital Adequacy to Risk Weighted Assets									0.105		0.105
Common Stock, Dividends, Per Share, Declared (in dollars per share)				$ 0.30		$ 0.29		$ 0.29
Common Stock, Dividends, Per Share, Cash Paid (in dollars per share)			$ 0.30		$ 0.29		$ 0.29		$ 0.30	$ 0.28	$ 0.88	$ 0.84
Stock Repurchase Program, Authorized Amount													$ 20.0		$ 20.0
Stock Repurchased During Period, Shares (in shares)											636,000
Stock Repurchased During Period, Value											$ 19.8
Treasury Stock Acquired, Average Cost Per Share (in dollars per share)											$ 31.14
Stock Repurchase Program, Remaining Authorized Repurchase Amount									$ 8.4		$ 8.4
Forecast [Member]
Common Stock, Dividends, Per Share, Cash Paid (in dollars per share)	$ 0.30
Subsequent Event [Member]
Common Stock, Dividends, Per Share, Declared (in dollars per share)		$ 0.30