Benefits and Pension Plans - Weighted Average Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2015
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)	3.30%	2.90%	4.16%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	2.90%	4.16%	3.83%
Expected long term return on plan assets (as a percent)	6.50%	6.50%	7.00%
Additional net periodic pension cost from each 100 basis point reduction in the discount rate	$ 51
Additional net periodic pension cost from each 100 basis point reduction in the expected return on plan assets	$ 69
Supplemental plan
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	2.73%	2.97%	3.22%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	2.97%	3.22%	3.01%
Additional net periodic pension cost from each 100 basis point reduction in the discount rate	$ 39
Neptco plan
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	2.95%	2.55%	4.30%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	2.55%	4.30%	4.06%
Expected long term return on plan assets (as a percent)	6.50%	6.50%	7.00%