Note 6 - Securities Sold Under Agreements to Repurchase - Securities Sold Under Agreement to Repurchase (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Securities sold under agreements to repurchase	$ 111,235,000	$ 103,519,000
Average daily balance during the period	98,923,000	101,005,000
Maximum daily balance during the period	$ 119,161,000	$ 123,046,000
Securities Sold under Agreements to Repurchase [Member]
Average interest rate at end of period	0.25%	0.26%
Average interest rate during the period	0.25%	0.24%