Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Aug. 31, 2021	Aug. 31, 2020	Aug. 31, 2019
Other Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 1,186	$ 1,050	$ 1,053
Interest costs	493	747	1,094
Expected return on assets	0	0	0
Settlement of retiree obligations	0	0	0
Prior service cost (credit) amortization	(445)	(445)	(556)
Actuarial loss (gain) amortization	(1,365)	(1,392)	(1,627)
Periodic benefit costs, net	$ (131)	$ (40)	$ (36)
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	2.43%	2.89%	4.08%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	2.57%	2.43%	2.89%
Qualified Pension Benefits | Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 45,229	$ 42,151	$ 38,592
Interest costs	16,563	21,722	28,396
Expected return on assets	(43,641)	(46,684)	(44,968)
Settlement of retiree obligations	0	0	51
Prior service cost (credit) amortization	178	178	190
Actuarial loss (gain) amortization	21,790	21,583	12,348
Periodic benefit costs, net	$ 40,119	$ 38,950	$ 34,609
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	2.65%	3.06%	4.23%
Expected return on plan assets	4.90%	5.50%	5.50%
Rate of compensation increase	4.99%	5.28%	5.14%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	2.78%	2.67%	3.06%
Rate of compensation increase	4.79%	4.99%	5.28%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Weighted-Average Interest Crediting Rate	4.65%	4.65%	4.65%
Nonqualified Pension Benefits | Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 433	$ 405	$ 311
Interest costs	273	429	747
Expected return on assets	0	0	0
Settlement of retiree obligations	0	0	191
Prior service cost (credit) amortization	(114)	(114)	(75)
Actuarial loss (gain) amortization	212	98	2
Periodic benefit costs, net	$ 804	$ 818	$ 1,176
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	2.07%	2.70%	4.09%
Rate of compensation increase	4.99%	5.28%	5.14%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	2.08%	2.15%	2.70%
Rate of compensation increase	4.79%	4.99%	5.28%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Weighted-Average Interest Crediting Rate	4.65%	4.65%	4.65%