Note 11 - Derivative Financial Instruments - Derivative Financial Instruments (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest rate swaps, notional amount		$ 739,262	$ 739,726
Interest rate swaps, net carrying value	[1]	26,270	3,630
Designated as Hedging Instrument [Member]
Interest rate swaps, notional amount		261,400	261,900
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Interest rate swaps, notional amount		441,500	441,500	$ 0
Not Designated as Hedging Instrument [Member]
Interest rate swaps, notional amount		36,300	36,300
Interest Rate Swaps 1 [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Interest rate swaps, notional amount		259,747	199,341
Interest rate swaps, net carrying value	[1]	15,677	6,971
Interest Rate Swaps 1 [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Interest rate swaps, notional amount		441,500	250,000
Interest rate swaps, net carrying value	[1]	11,721	417
Interest Rate Swaps 2 [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Interest rate swaps, notional amount		1,694	62,564
Interest rate swaps, net carrying value	[1]	(12)	(921)
Interest Rate Swaps 2 [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Interest rate swaps, notional amount			191,500
Interest rate swaps, net carrying value	[1]		(7)
Interest Rate Swap [Member]
Interest rate swaps, notional amount		279,800	280,200
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Interest rate swaps, notional amount		36,321	36,321
Interest rate swaps, net carrying value	[1]	$ (1,116)	$ (2,830)

[1] Derivatives in a net positive position are recorded as "Other assets" and derivatives in a net negative position are recorded as "Other liabilities" in the Consolidated Statements of Financial Condition.