Note 14 - Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Balance	$ 1,690,963	$ 1,461,277	$ 1,462,240
Other comprehensive income (loss)	5,159	(17,807)	11,229
Balance	1,692,423	1,690,963	1,461,277
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Balance	(13)	3,719	1,454
Other comprehensive income (loss)	0	(2,443)	2,265
Impact of Adoption of Accounting Standard Update	(1,300)
Balance	(13)	(13)	3,719
Balance, tax	0	1,791	627
Change, tax	0	(1,791)	1,164
Balance, tax	0	0	1,791
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] | Accounting Standards Update 2016-01 [Member]
Impact of Adoption of Accounting Standard Update		(1,289)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Balance	(8,784)	5,030	(5,260)
Other comprehensive income (loss)	8,436	(13,814)	10,290
Impact of Adoption of Accounting Standard Update		0
Balance	(348)	(8,784)	5,030
Balance, tax	0	0	0
Change, tax	0	0	0
Balance, tax	0	0	0
Accumulated Defined Benefit Plans Adjustment, Net Transition Attributable to Parent [Member]
Balance	(33,672)	(32,122)	(30,796)
Other comprehensive income (loss)	(3,277)	(1,550)	(1,326)
Impact of Adoption of Accounting Standard Update		0
Balance	(36,949)	(33,672)	(32,122)
Balance, tax	12,575	10,120	10,120
Change, tax	0	2,455	0
Balance, tax	12,575	12,575	10,120
AOCI Attributable to Parent [Member]
Balance	(42,469)	(23,373)	(34,602)
Other comprehensive income (loss)	5,159	(17,807)	11,229
Impact of Adoption of Accounting Standard Update		(1,289)
Balance	(37,310)	(42,469)	(23,373)
Balance, tax	12,575	11,911	10,747
Change, tax	0	664	1,164
Balance, tax	$ 12,575	$ 12,575	$ 11,911