Note 7 - Investment Securities (Details Textual) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale, Total	$ 1,283,060,000	$ 1,283,060,000			$ 1,333,626,000
Proceeds from Sale of Mortgage-backed Securities (MBS), Available-for-sale		0	$ 0
Proceeds from Maturities, Prepayments and Calls of Mortgage Backed Securities (MBS)		79,400,000	48,500,000
Proceeds From Sales And Repayments Of Other Investment Securities		99,600,000	99,500,000
Proceeds from Maturities Repayments and Calls of Other Investment Securities		266,900,000	341,300,000
Realized Loss From Sale And Call Of Investment Securities		14,000	438,000
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Total		0	0
Equity Securities, FV-NI, Unrealized Gain (Loss), Total	391,000	(4,580,000)
Equity Securities, FV-NI	23,522,000	23,522,000			28,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss, Total	38,900,000	38,900,000			16,684,000
Pledged Investment Securities	$ 33,100,000	$ 33,100,000			$ 272,200,000
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption				$ 8,600,000