Benefit Plans - Narrative (Details) $ in Thousands	12 Months Ended
	Aug. 31, 2021 USD ($) Employers	Aug. 31, 2020 USD ($)	Aug. 31, 2019 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 23,200
Defined benefit plan, health care benefits annual rate of increase in the per capita cost	6.80%
Defined benefit plan, health care benefits, decrease in the per capita cost trend rate	4.50%
Defined contribution plan, contributions by employer	$ 30,100	$ 34,500	$ 31,000
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	$ 32,000
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	877,900	871,600
Defined benefit plan, expected future benefit payments, next twelve months	70,100
Non-Qualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	20,500	18,200
Defined benefit plan, expected future benefit payments, next twelve months	2,220
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	1,763	$ 1,568
Defined benefit plan, expected future benefit payments, next twelve months	1,970
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	$ 4,200
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	80.00%
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	20.00%
Minimum | Co-op Retirement Plan | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, funded percentage	80.00%