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Retirement Plans (Plan's Allocated Target Ranges and Actual Weighted Average Asset Allocation by Investment Categories) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Actual percentage	100.00%	100.00%
Maximum percentage of equity security value of total plans portfolio value that is allowed	8.00%
Maximum percentage of any corporation outstanding shares that is allowed for retirement income plan equity portfolio	5.00%
Return Seeking Assets [Member]
Actual percentage	58.00%	50.00%
Liability Hedging Assets [Member]
Actual percentage	42.00%	50.00%
Minimum [Member] | Return Seeking Assets [Member]
Defined Benefit Plan, Target Allocation Percentage	0.2
Minimum [Member] | Liability Hedging Assets [Member]
Defined Benefit Plan, Target Allocation Percentage	0.4
Maximum [Member] | Return Seeking Assets [Member]
Defined Benefit Plan, Target Allocation Percentage	0.6
Maximum [Member] | Liability Hedging Assets [Member]
Defined Benefit Plan, Target Allocation Percentage	0.8

X

- Definition

The maximum percentage of outstanding shares of any one corporation that is allowed for the Retirement Income Plan.

+ References

No definition available.

+ Details

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X

- Definition

The maximum percentage of any individual security value of the total Retirement Income Plan's portfolio value that is allowed.

+ References

No definition available.

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X

- Definition

Description of target allocation percentages, or range of percentages, considering the major categories of plan assets as of the latest statement of financial position presented (on a weighted-average basis for employers with more than one plan) that are pertinent to an understanding of the plan investment policies and strategies.

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)(i)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920

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- Definition

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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