Benefit Plans - Narative (Details) $ in Thousands	12 Months Ended
	Aug. 31, 2017 USD ($) Employers	Aug. 31, 2016 USD ($)	Aug. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, health care benefits annual rate of increase in the per capita cost	7.90%
Defined benefit plan, health care benefits, decrease in the per capita cost trend rate	4.50%
Defined contribution plan, contributions by employer	$ 19,900	$ 29,500	$ 27,400
Pension protection act, percentage of employers that are rural cooperatives or cooperative organizations owned by agricultural producers, criteria	85.00%
Qualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 740,900	766,200
Defined benefit plan, expected future benefit payments, next twelve months	$ 64,044
Qualified Pension Benefits | Multiemployer Plans, Pension | Co-op Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Number of employers contributing to Co-op Retirement Plan (in number of employers) | Employers	400
Non-Qualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 20,600	$ 23,700
Defined benefit plan, expected future benefit payments, next twelve months	2,270
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected future benefit payments, next twelve months	2,140
Pension Plans And Postretirement Plans, Defined Benefit, Non-Qualified [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected future benefit payments, next twelve months	$ 4,400
Maximum | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage	55.00%
Maximum | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage	65.00%
Maximum | Qualified Pension Benefits | Multiemployer Plans, Pension | Co-op Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions as percent of total contributions of all contributing employers	5.00%
Minimum | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage	35.00%
Minimum | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage	45.00%
Minimum | Qualified Pension Benefits | Multiemployer Plans, Pension | Co-op Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, funded percentage	80.00%