Retirement Plans, Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rate (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation[Abstract]
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Increase	$ 0	$ 0
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Decrease	0	0
SERP [Member]
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation[Abstract]
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Increase	0	0
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Decrease	0	0
Other Postretirement Benefits [Member]
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation[Abstract]
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Increase	75	74
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Decrease	(65)	(64)
Effect of one-percentage-point change in assumed health care cost trend rate on aggregate service and interest cost, Increase	3	4	$ 5
Effect of one-percentage-point change in assumed health care cost trend rate on aggregate service and interest cost, Decrease	$ (3)	$ (3)	$ (4)

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- Definition

Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate.

+ 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 (m)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount of decrease in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point decrease in assumed health care cost trend rate.

+ References

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- Definition

Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate.

+ References

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- Definition

Amount of increase in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point increase in assumed health care cost trend rate.

+ References

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