Note 4 - Investment Securities (Details Textual)	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)
Proceeds from Sale of Mortgage-backed Securities (MBS), Available-for-sale	$ 0	$ 0
Proceeds from Maturities, Prepayments and Calls of Mortgage Backed Securities (MBS)	105,900,000	71,600,000
Proceeds From Sales And Repayments Of Other Investment Securities	99,900,000	111,700,000
Proceeds from Maturities Repayments and Calls of Other Investment Securities	341,900,000	491,000,000
Realized Gain From Sale And Call Of Investment Securities	36,000	1,700,000
Realized Loss From Sale And Call Of Investment Securities	$ 14,000	$ 710,000
Temporarily Impaired Securities As Percentage Of Investment Securities	84.90%
Percentage Of Unrealized Losses For Securities With Unrealized Losses For Less Than Twelve Months	0.40%
Percentage Of Unrealized Losses For Securities With Unrealized Losses For Greater Than Twelve Months	2.80%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	90	24
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	14	63
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax, Total	$ 27,000,000
Pledged Investment Securities	28,500,000	$ 272,200,000
Equity Securities, FV-NI, Unrealized Gain (Loss), Total	(2,787,000)
Equity Securities, FV-NI	25,098,000	$ 28,100,000
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 8,600,000			$ 8,600,000