Note 15 - Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Balance	$ 1,692,423	$ 1,690,963	$ 1,461,277
Impact of Adoption of Accounting Standard Update			0
Other comprehensive income (loss)	4,421	5,159	(17,807)
Balances	1,702,340	1,692,423	1,690,963
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Balance	(13)	(13)	3,719
Impact of Adoption of Accounting Standard Update			(1,300)
Other comprehensive income (loss)	0	0	(2,443)
Balances	(13)	(13)	(13)
Balance, tax	0	0	1,791
Change, tax	0	0	(1,791)
Balance, tax	0	0	0
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	(348)	(8,784)	5,030
Impact of Adoption of Accounting Standard Update			0
Other comprehensive income (loss)	7,980	8,436	(13,814)
Balances	7,632	(348)	(8,784)
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	(36,949)	(33,672)	(32,122)
Impact of Adoption of Accounting Standard Update			0
Other comprehensive income (loss)	(3,559)	(3,277)	(1,550)
Balances	(40,508)	(36,949)	(33,672)
Balance, tax	12,575	12,575	10,120
Change, tax	0	0	2,455
Balance, tax	12,575	12,575	12,575
AOCI Attributable to Parent [Member]
Balance	(37,310)	(42,469)	(23,373)
Impact of Adoption of Accounting Standard Update			(1,289)
Other comprehensive income (loss)	4,421	5,159	(17,807)
Balances	(32,889)	(37,310)	(42,469)
Balance, tax	12,575	12,575	11,911
Change, tax	0	0	664
Balance, tax	$ 12,575	$ 12,575	$ 12,575