Retirement Plans (Impact of One Percentage Point Change in Assumed Health Care and Dental Cost Trend Rates) (Details) - Postretirement Plan $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Effect of one percentage point increase on service and interest cost components	$ 8
Effect of one percentage point decrease on service and interest cost components	(8)
Effect of one percentage point increase on postretirement benefit obligation	196
Effect of one percentage point decrease on postretirement benefit obligation	$ (177)

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

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

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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.

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