Note 11 - Derivative Financial Instruments (Details Textual) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Junior Subordinated Notes, Total	$ 40,151		$ 40,151		$ 36,986
Derivative Asset, Notional Amount	747,555		747,555		739,726
Short-term Debt, Total	441,500		441,500		441,500
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	100		200
Derivative, Net Hedge Ineffectiveness Gain (Loss)	0	$ 0	0	$ 0
Not Designated as Hedging Instrument [Member]
Derivative Asset, Notional Amount	36,300		36,300		36,300
Designated as Hedging Instrument [Member]
Derivative Asset, Notional Amount	269,700		269,700		261,900
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative Asset, Notional Amount	441,500		441,500		441,500	$ 0
Interest Rate Swap [Member]
Derivative Asset, Notional Amount	288,100		288,100		280,200
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative Asset, Notional Amount	36,321		36,321		36,321
Floating Rate Junior Subordinated Debentures [Member]
Derivative, Amount of Hedged Item	18,000		18,000		18,000
Junior Subordinated Notes, Total	$ 61,900		$ 61,900		$ 61,900