Note 11 - Postretirement Medical Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Curtailment gain recognized		$ (417)
Postretirement Health Coverage [Member]
Service cost	$ 140	146
Interest cost	153	167
Curtailment gain recognized	0	(417)
Recognized prior service cost	0	75
Net periodic benefit cost	$ 293	$ (29)

Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) 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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Amount of gain (loss) recognized in net periodic benefit (cost) credit from event reducing expected years of future service of present employees or eliminating accrual of defined benefits for some or all future services of present employees.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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