Benefit Plans - Rollforward of Benefit Obligation and Plan Assets Funded (Details) - USD ($) $ in Thousands	12 Months Ended
Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2015
Qualified Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 42,149	$ 37,533	$ 36,006
Interest costs	22,999	30,773	28,046
Expected return on assets	(48,235)	(48,055)	(49,746)
Settlement of retiree obligations	0	0	0
Prior service cost (credit) amortization	1,540	1,606	1,631
Actuarial loss (gain) amortization	22,869	19,016	19,621
Periodic benefit costs, net	$ 41,322	$ 40,873	$ 35,558
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.60%	4.20%	4.00%
Expected return on plan assets	5.75%	6.00%	6.50%
Rate of compensation increase	5.60%	4.90%	4.90%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	3.80%	3.60%	4.20%
Rate of compensation increase	5.10%	5.60%	4.90%
Non-Qualified Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 1,206	$ 1,035	$ 875
Interest costs	843	1,406	1,414
Expected return on assets	0	0	0
Settlement of retiree obligations	(30)	0	1,635
Prior service cost (credit) amortization	19	228	228
Actuarial loss (gain) amortization	546	692	1,058
Periodic benefit costs, net	$ 2,584	$ 3,361	$ 5,210
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.30%	4.20%	4.00%
Rate of compensation increase	5.60%	4.90%	5.15%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	3.55%	3.30%	4.50%
Rate of compensation increase	5.10%	5.60%	4.80%
Other Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 1,160	$ 1,412	$ 1,513
Interest costs	930	1,709	1,489
Expected return on assets	0	0	0
Settlement of retiree obligations	0	0	0
Prior service cost (credit) amortization	(565)	(120)	(426)
Actuarial loss (gain) amortization	(798)	(464)	(431)
Periodic benefit costs, net	$ 727	$ 2,537	$ 2,145
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.30%	4.20%	4.20%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	3.60%	3.30%	3.75%

Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan.

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- Definition

Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan.

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

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Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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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.

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- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

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Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan.

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- Definition

Amount of cost recognized for passage of time related to 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 (h)(2)
-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 1
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Amount of net periodic benefit cost (credit) for defined benefit plan.

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Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement.

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- Definition

Amount of cost for actuarial present value of benefits attributed to service rendered by employee for 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 (h)(1)
-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 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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