Benefit Plans - Narrative (Details) $ in Thousands	12 Months Ended
	Aug. 31, 2020 USD ($) Employers	Aug. 31, 2019 USD ($)	Aug. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 46,400
Defined benefit plan, health care benefits annual rate of increase in the per capita cost	7.10%
Defined benefit plan, health care benefits, decrease in the per capita cost trend rate	4.50%
Defined contribution plan, contributions by employer	$ 34,500	$ 31,000	$ 24,700
Pension protection act, percentage of employers that are rural cooperatives or cooperative organizations owned by agricultural producers, criteria	85.00%
Separate Account, Liability, Surrender and Withdrawal	$ 46,000
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	871,600	833,200
Defined benefit plan, expected future benefit payments, next twelve months	75,700
Non-Qualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	18,200	16,900
Defined benefit plan, expected future benefit payments, next twelve months	1,660
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	1,568	$ 1,641
Defined benefit plan, expected future benefit payments, next twelve months	2,090
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	$ 3,800
Co-op Retirement Plan | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Number of employers contributing to Co-op Retirement Plan (in number of employers) | Employers	400
Maximum | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage	65.00%
Maximum | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage	55.00%
Maximum | Co-op Retirement Plan | Pension Benefits
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	45.00%
Minimum | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage	35.00%
Minimum | Co-op Retirement Plan | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, funded percentage	80.00%