Pension Benefit and Retirement Health and Life Insurance Benefits (One-Percentage Point Change in Assumed Health Care Cost Trend Rates) (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($)
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]
Effect on total service and interest cost - Increase	$ 9
Effect on total service and interest cost - Decrease	(9)
Effect on other postretirement benefit obligations - Increase	68
Effect on other postretirement benefit obligations - Decrease	$ (63)

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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

+ Details

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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

+ Details

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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

+ Details

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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

+ Details

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