Employee Retirement Plans - Allocation of Pension Plan and VEBA Trust Assets (Details)	Sep. 30, 2017	Sep. 30, 2016
VEBA Trust
Defined Benefit Plan Disclosure
Actual pension plan	100.00%	100.00%
Target asset allocation	100.00%
VEBA Trust \| Domestic
Defined Benefit Plan Disclosure
Actual pension plan	63.10%	69.90%
Target asset allocation	65.00%
VEBA Trust \| Domestic \| Minimum
Defined Benefit Plan Disclosure
Target asset allocation	60.00%
VEBA Trust \| Domestic \| Maximum
Defined Benefit Plan Disclosure
Target asset allocation	70.00%
VEBA Trust \| Fixed income funds & cash equivalents
Defined Benefit Plan Disclosure
Actual pension plan	36.90%	30.10%
Target asset allocation	35.00%
VEBA Trust \| Fixed income funds & cash equivalents \| Minimum
Defined Benefit Plan Disclosure
Target asset allocation	30.00%
VEBA Trust \| Fixed income funds & cash equivalents \| Maximum
Defined Benefit Plan Disclosure
Target asset allocation	40.00%
Pension Benefits
Defined Benefit Plan Disclosure
Actual pension plan	100.00%	100.00%
Target asset allocation	100.00%
Pension Benefits \| Equity investments
Defined Benefit Plan Disclosure
Actual pension plan	67.60%	64.30%
Target asset allocation	65.00%
Pension Benefits \| Equity investments \| Minimum
Defined Benefit Plan Disclosure
Target asset allocation	60.00%
Pension Benefits \| Equity investments \| Maximum
Defined Benefit Plan Disclosure
Target asset allocation	70.00%
Pension Benefits \| Domestic
Defined Benefit Plan Disclosure
Actual pension plan	55.20%	54.10%
Target asset allocation	52.50%
Pension Benefits \| Domestic \| Minimum
Defined Benefit Plan Disclosure
Target asset allocation	40.00%
Pension Benefits \| Domestic \| Maximum
Defined Benefit Plan Disclosure
Target asset allocation	65.00%
Pension Benefits \| International
Defined Benefit Plan Disclosure
Actual pension plan	12.40%	10.20%
Target asset allocation	12.50%
Pension Benefits \| International \| Minimum
Defined Benefit Plan Disclosure
Target asset allocation	7.50%
Pension Benefits \| International \| Maximum
Defined Benefit Plan Disclosure
Target asset allocation	17.50%
Pension Benefits \| Fixed income funds & cash equivalents
Defined Benefit Plan Disclosure
Actual pension plan	32.40%	35.70%
Target asset allocation	35.00%
Pension Benefits \| Fixed income funds & cash equivalents \| Minimum
Defined Benefit Plan Disclosure
Target asset allocation	30.00%
Pension Benefits \| Fixed income funds & cash equivalents \| Maximum
Defined Benefit Plan Disclosure
Target asset allocation	40.00%

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- Definition

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one 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 (d)(5)(i)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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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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