Retirement Plans, Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost, Other Postretirement Benefits (Details) - Other Postretirement Benefits [Member] - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted average assumptions used to determine net periodic benefit cost [Abstract]
Discount rate	4.20%	4.50%	4.10%
Health care cost trend rate [Abstract]
Initial rate	7.00%	6.50%	7.00%
Ultimate rate	5.00%	5.00%	5.00%
Years to ultimate	2021	2019	2019
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 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)

Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Initial Health Care Cost Trend Rate

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- Definition

Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Ultimate Health Care Cost Trend Rate

+ References

No definition available.

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Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Year That Reaches ultimate Health Care Cost Trend Rate.

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Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

+ 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 (k)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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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

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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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

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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