Retirement Plans, Summary of Expected Federal Subsidy (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Expected federal subsidy [Abstract]
Year one	$ 0	$ 0
Year two	0	0
Year three	0	0
Year four	0	0
Year five	0	0
Next five years	0	0
SERP [Member]
Expected federal subsidy [Abstract]
Year one	0	0
Year two	0	0
Year three	0	0
Year four	0	0
Year five	0	0
Next five years	0	0
Other Postretirement Benefits [Member]
Expected federal subsidy [Abstract]
Year one	(7)	(7)
Year two	(6)	(7)
Year three	(6)	(7)
Year four	(6)	(7)
Year five	(7)	(7)
Next five years	$ (30)	$ (33)

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Amount of prescription drug subsidy receipts expected after fifth fiscal year following latest fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 60
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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-Section 50
-Paragraph 1
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Amount of prescription drug subsidy receipts expected in next fiscal year following latest fiscal year.

+ References

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

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-Publisher FASB
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-Topic 715
-SubTopic 60
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964

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Amount of prescription drug subsidy receipts expected in fifth fiscal year following latest fiscal year.

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Amount of prescription drug subsidy receipts expected in fourth fiscal year following latest fiscal year.

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Amount of prescription drug subsidy receipts expected in third fiscal year following latest fiscal year.

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Amount of prescription drug subsidy receipts expected in second fiscal year following latest fiscal year.

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