Benefit Plans - Components of Net Periodic Benefit Costa and Amounts Recognized in Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Aug. 31, 2020	Aug. 31, 2019	Aug. 31, 2018
Pension Benefits | Qualified Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service cost	$ 0	$ 18	$ 244
Net actuarial loss (gain)	3,401	47,556	(8,553)
Amortization of actuarial (gain) loss	21,583	12,307	18,073
Amortization of prior service (credit) costs	(178)	(190)	(1,437)
Settlement of retiree obligations	0	0	0
Total recognized in other comprehensive loss (income)	(18,360)	35,077	(27,819)
Pension Benefits | Nonqualified Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service cost	0	0	0
Net actuarial loss (gain)	2,157	1,917	(578)
Amortization of actuarial (gain) loss	98	2	61
Amortization of prior service (credit) costs	114	75	(30)
Settlement of retiree obligations	(397)	(191)	112
Total recognized in other comprehensive loss (income)	1,776	1,799	(557)
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service cost	0	0	0
Net actuarial loss (gain)	(1,011)	801	(2,234)
Amortization of actuarial (gain) loss	1,392	1,627	1,224
Amortization of prior service (credit) costs	445	556	565
Settlement of retiree obligations	0	0	0
Total recognized in other comprehensive loss (income)	$ 826	$ 2,984	$ (445)