Note 9 - Pension and Other Postretirement Benefit Plans (Details Textual) - USD ($)	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Directors' Plan [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		$ 200,000
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 24,000
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		$ 200,000
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 20,000

Amount of contribution received by defined benefit plan from employer which increases plan assets.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(ii)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e2709-114920

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration