Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Aug. 31, 2020	Aug. 31, 2019	Aug. 31, 2018
Other Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 1,050	$ 1,053	$ 943
Interest costs	747	1,094	908
Expected return on assets	0	0	0
Settlement of retiree obligations	0	0	0
Prior service cost (credit) amortization	(445)	(556)	(565)
Actuarial loss (gain) amortization	(1,392)	(1,627)	(1,224)
Periodic benefit costs, net	$ (40)	$ (36)	$ 62
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	2.89%	4.08%	3.56%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	2.43%	2.89%	4.13%
Qualified Pension Benefits | Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 42,151	$ 38,592	$ 39,677
Interest costs	21,722	28,396	24,007
Expected return on assets	(46,684)	(44,968)	(48,159)
Settlement of retiree obligations	0	51	0
Prior service cost (credit) amortization	178	190	1,437
Actuarial loss (gain) amortization	21,583	12,348	18,073
Periodic benefit costs, net	$ 38,950	$ 34,609	$ 35,035
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.06%	4.23%	3.80%
Expected return on plan assets	5.50%	5.50%	5.75%
Rate of compensation increase	5.28%	5.14%	5.08%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	2.67%	3.06%	4.23%
Rate of compensation increase	4.99%	5.28%	5.14%
Nonqualified Pension Benefits | Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 405	$ 311	$ 548
Interest costs	429	747	711
Expected return on assets	0	0	0
Settlement of retiree obligations	0	191	(112)
Prior service cost (credit) amortization	(114)	(75)	30
Actuarial loss (gain) amortization	98	2	61
Periodic benefit costs, net	$ 818	$ 1,176	$ 1,238
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	2.70%	4.09%	3.53%
Rate of compensation increase	5.28%	5.14%	5.08%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	2.15%	2.70%	4.09%
Rate of compensation increase	4.99%	5.28%	5.14%