Note 6 - Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plan [Member]
Service cost	$ 655	$ 709	$ 2,078	$ 2,127
Interest cost	599	659	1,918	1,981
Expected return on plan assets	(1,138)	(982)	(3,525)	(2,947)
Recognized actuarial loss (gain)	436	525	1,384	1,576
Settlement loss	448		3,841
Net periodic benefit cost	1,000	911	5,696	2,737
Postretirement Health Coverage [Member]
Service cost	312	298	937	894
Interest cost	203	210	610	631
Expected return on plan assets
Recognized actuarial loss (gain)	(168)	(174)	(505)	(523)
Settlement loss
Net periodic benefit cost	$ 347	$ 334	$ 1,042	$ 1,002

Amount, before tax, after reclassification adjustment, of increase (decrease) in accumulated other comprehensive income from gain (loss) of defined benefit plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580

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Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of net periodic benefit cost (credit) for defined benefit plan.

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