Note 4 - Investment Securities (Details Textual)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Proceeds from Sale of Mortgage Backed Securities (MBS) categorized as Available-for-sale	$ 0	$ 605,200,000
Proceeds from Maturities, Prepayments and Calls of Mortgage Backed Securities (MBS)	71,600,000	153,000,000
Proceeds From Sales And Repayments Of Other Investment Securities	111,700,000	294,000
Proceeds from Maturities Repayments and Calls of Other Investment Securities	491,000,000	460,000,000
Realized Gain From Sale And Call Of Investment Securities	1,700,000	5,100,000
Realized Loss From Sale And Call Of Investment Securities	$ 710,000	$ 0
Temporarily Impaired Securities As Percentage Of Investment Securities	88.60%
Percentage Of Unrealized Losses For Securities With Unrealized Losses For Less Than Twelve Months	0.60%
Percentage Of Unrealized Losses For Securities With Unrealized Losses For Greater Than Twelve Months	2.10%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	24	7
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	63	30
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 16,684,000	$ 14,968,000
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities	0
Pledged Investment Securities	$ 272,200,000	$ 649,100,000