Benefits and Pension Plans - Asset Categories (Details)	12 Months Ended
Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2015
Qualified Plan
Target allocation and weighted-average asset allocations
Total target allocation (as a percent)	100.00%	100.00%	100.00%
Percentage of Plan Assets	100.00%	100.00%	100.00%
Expected long term return on plan assets (as a percent)	6.50%	6.50%	7.00%
Neptco plan
Target allocation and weighted-average asset allocations
Total target allocation (as a percent)	100.00%	100.00%	100.00%
Percentage of Plan Assets	100.00%	100.00%	100.00%
Expected long term return on plan assets (as a percent)	6.50%	6.50%	7.00%
Equity securities \| Qualified Plan
Target allocation and weighted-average asset allocations
Target Allocation Range, minimum (as a percent)	10.00%	10.00%	10.00%
Target Allocation Range, maximum (as a percent)	80.00%	80.00%	80.00%
Percentage of Plan Assets	39.00%	46.00%	44.00%
Equity securities \| Neptco plan
Target allocation and weighted-average asset allocations
Target Allocation Range, minimum (as a percent)	10.00%	10.00%	10.00%
Target Allocation Range, maximum (as a percent)	80.00%	80.00%	80.00%
Percentage of Plan Assets	43.00%	43.00%	41.00%
Debt securities \| Qualified Plan
Target allocation and weighted-average asset allocations
Target Allocation Range, minimum (as a percent)	20.00%	20.00%	20.00%
Target Allocation Range, maximum (as a percent)	70.00%	70.00%	70.00%
Percentage of Plan Assets	61.00%	54.00%	56.00%
Debt securities \| Neptco plan
Target allocation and weighted-average asset allocations
Target Allocation Range, minimum (as a percent)	20.00%	20.00%	20.00%
Target Allocation Range, maximum (as a percent)	70.00%	70.00%	70.00%
Percentage of Plan Assets	51.00%	50.00%	53.00%
Other \| Qualified Plan
Target allocation and weighted-average asset allocations
Target Allocation Range, minimum (as a percent)	0.00%	0.00%	0.00%
Target Allocation Range, maximum (as a percent)	100.00%	100.00%	100.00%
Other \| Neptco plan
Target allocation and weighted-average asset allocations
Target Allocation Range, minimum (as a percent)	0.00%	0.00%	0.00%
Target Allocation Range, maximum (as a percent)	100.00%	100.00%	100.00%
Percentage of Plan Assets	6.00%	7.00%	6.00%

An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Expected Long-Term Rate of Return on Plan Assets
-URI http://asc.fasb.org/extlink&oid=6512046

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
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Target allocation of investments of this type to total plan assets presented on a weighted average basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
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Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis.

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Target allocation minimum percentage of investments of this type to total plan assets presented on a weighted average basis.

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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
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