Benefits and Pension Plans - Weighted Average Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Aug. 31, 2020	Aug. 31, 2019	Aug. 31, 2018
Weighted-average assumptions used to determine benefit obligations
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
Weighted-average assumptions used to determine net periodic benefit cost
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
Qualified Plan
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	1.92%	2.58%	3.80%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	2.58%	3.80%	3.30%
Expected long term return on plan assets (as a percent)	5.60%	5.40%	5.40%
Reduction in discount rate used to estimate additional net periodic pension cost (as a percent)	1.00%
Additional net periodic pension cost from each 100 basis point reduction in the discount rate	$ 41
Reduction in expected return on plan assets used to estimate the increase in net periodic pension cost (as a percent)	1.00%
Additional net periodic pension cost from each 100 basis point reduction in the expected return on plan assets	$ 68
Supplemental plan
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	1.65%	2.37%	3.57%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	2.37%	3.57%	2.73%
Additional net periodic pension cost from each 100 basis point reduction in the discount rate	$ (57)
NEPTCO | Qualified Plan
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)		2.29%	3.59%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	2.29%	3.59%	2.95%
Expected long term return on plan assets (as a percent)	5.60%	5.40%	5.20%