Note 6 - Accrued Retirement Benefits - Accrued Retirement Benefits (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued retirement benefits	$ 10,036	$ 8,031
Less current portion	(165)	(164)
Non-current portion of accrued retirement benefits	9,871	7,867
Pension Plan [Member] \| Qualified Plan [Member]
Accrued retirement benefits	7,971	5,812
Supplemental Employee Retirement Plan [Member] \| Nonqualified Plan [Member]
Accrued retirement benefits	$ 2,065	$ 2,219

Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(15))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
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-URI http://asc.fasb.org/extlink&oid=109237824&loc=d3e1703-114919

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(15))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 50
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-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant