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, 2018	Aug. 31, 2017	Aug. 31, 2016
Pension Benefits | Qualified Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service cost (credit)	$ 244	$ 0	$ 411
Net actuarial loss (gain)	(8,553)	(16,044)	17,712
Amortization of actuarial loss (gain)	(18,073)	(22,869)	(19,016)
Amortization of prior service costs (credit)	(1,437)	(1,540)	(1,606)
Settlement of retiree obligations	0	0	0
Total recognized in other comprehensive income	(27,819)	(40,453)	(2,499)
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	(1,044)
Net actuarial loss (gain)	(578)	(6,345)	(655)
Amortization of actuarial loss (gain)	(61)	(546)	(692)
Amortization of prior service costs (credit)	(30)	(19)	(228)
Settlement of retiree obligations	112	30	0
Total recognized in other comprehensive income	(557)	(6,880)	(2,619)
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service cost (credit)	0	0	(4,495)
Net actuarial loss (gain)	(2,234)	(5,427)	(2,290)
Amortization of actuarial loss (gain)	1,224	798	464
Amortization of prior service costs (credit)	565	565	120
Settlement of retiree obligations	0	0	0
Total recognized in other comprehensive income	$ (445)	$ (4,064)	$ (6,201)