Note 6 - Securities Sold Under Agreements to Repurchase - Securities Sold Under Agreement to Repurchase (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Securities sold under agreements to repurchase	$ 104,894,000	$ 118,748,000
Average interest rate at end of period	0.25%	0.16%
Average daily balance during the period	$ 108,048,000	$ 116,587,000
Average interest rate during the period	0.22%	0.16%
Maximum daily balance during the period	$ 123,046,000	$ 142,459,000