Note 7 - Investment Securities (Details Textual) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale, Total	$ 1,475,949,000		$ 1,475,949,000			$ 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)			50,400,000	28,600,000
Proceeds From Sales And Repayments Of Other Investment Securities			0	99,500,000
Proceeds from Maturities Repayments and Calls of Other Investment Securities			206,900,000	170,000,000
Realized Loss From Sale And Call Of Investment Securities			0	438,000
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Total			0	25,000
Equity Securities, FV-NI, Unrealized Gain (Loss), Total	(1,124,000)		(4,971,000)
Equity Securities, FV-NI	23,131,000		23,131,000			28,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss, Total	35,200,000		35,200,000			16,684,000
Pledged Investment Securities	$ 158,100,000		$ 158,100,000			$ 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