Note 15 - Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance	$ 1,461,277	$ 1,462,240	$ 1,329,300
Change, net of tax	(17,807)	11,229	(1,971)
Balance	1,690,963	1,461,277	1,462,240
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Balance	3,719	1,454	(1,159)
Change, net of tax	(2,443)	2,265	2,613
Impact of Adoption of Accounting Standard Update	(1,300)
Balance	(13)	3,719	1,454
Balance, tax	1,791	627	631
Change, tax	(1,791)	1,164	(4)
Balance, tax		1,791	627
Accumulated Net Investment Gain (Loss) Attributable to 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	5,030	(5,260)
Change, net of tax	(13,814)	10,290	(5,260)
Impact of Adoption of Accounting Standard Update
Balance	(8,784)	5,030	(5,260)
Balance, tax
Change, tax
Balance, tax
Accumulated Defined Benefit Plans Adjustment, Net Transition Attributable to Parent [Member]
Balance	(32,122)	(30,796)	(31,472)
Change, net of tax	(1,550)	(1,326)	676
Impact of Adoption of Accounting Standard Update
Balance	(33,672)	(32,122)	(30,796)
Balance, tax	10,120	10,120	10,630
Change, tax	2,455		(510)
Balance, tax	12,575	10,120	10,120
AOCI Attributable to Parent [Member]
Balance	(23,373)	(34,602)	(32,631)
Change, net of tax	(17,807)	11,229	(1,971)
Impact of Adoption of Accounting Standard Update	(1,289)
Balance	(42,469)	(23,373)	(34,602)
Balance, tax	11,911	10,747	11,261
Change, tax	664	1,164	(514)
Balance, tax	$ 12,575	$ 11,911	$ 10,747