Note 14 - Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Balance	$ 1,479,844	$ 1,338,926	$ 1,396,974
Change, net of tax	11,229	(1,971)	(600)
Balance	1,483,882	1,479,844	1,338,926
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Balance	1,454	(1,159)	(1,409)
Change, net of tax	2,265	2,613	250
Balance	3,719	1,454	(1,159)
Balance, tax	627	631	625
Change, tax	1,164	(4)	6
Balance, tax	1,791	627	631
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Balance	(5,260)
Change, net of tax	10,290	(5,260)
Balance	5,030	(5,260)
Balance, tax
Change, tax
Balance, tax
Accumulated Defined Benefit Plans Adjustment, Net Transition Attributable to Parent [Member]
Balance	(30,796)	(31,472)	(30,622)
Change, net of tax	(1,326)	676	(850)
Balance	(32,122)	(30,796)	(31,472)
Balance, tax	10,120	10,630	10,417
Change, tax		(510)	213
Balance, tax	10,120	10,120	10,630
AOCI Attributable to Parent [Member]
Balance	(34,602)	(32,631)	(32,031)
Change, net of tax	11,229	(1,971)	(600)
Balance	(23,373)	(34,602)	(32,631)
Balance, tax	10,747	11,261	11,042
Change, tax	1,164	(514)	219
Balance, tax	$ 11,911	$ 10,747	$ 11,261