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, 2019	Aug. 31, 2018	Aug. 31, 2017
Pension Benefits | Qualified Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service cost (credit)	$ 18	$ 244	$ 0
Net actuarial loss (gain)	47,556	(8,553)	(16,044)
Amortization of actuarial loss (gain)	(12,307)	(18,073)	(22,869)
Amortization of prior service costs (credit)	(190)	(1,437)	(1,540)
Settlement of retiree obligations	0	0	0
Total recognized in other comprehensive income	35,077	(27,819)	(40,453)
Pension Benefits | Non-Qualified Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service cost (credit)	0	0	0
Net actuarial loss (gain)	1,917	(578)	(6,345)
Amortization of actuarial loss (gain)	(2)	(61)	(546)
Amortization of prior service costs (credit)	75	(30)	(19)
Settlement of retiree obligations	(191)	112	30
Total recognized in other comprehensive income	1,799	(557)	(6,880)
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service cost (credit)	0	0	0
Net actuarial loss (gain)	801	(2,234)	(5,427)
Amortization of actuarial loss (gain)	1,627	1,224	798
Amortization of prior service costs (credit)	556	565	565
Settlement of retiree obligations	0	0	0
Total recognized in other comprehensive income	$ 2,984	$ (445)	$ (4,064)