Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
Other Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 943	$ 1,160	$ 1,412
Interest costs	908	930	1,709
Expected return on assets	0	0	0
Settlement of retiree obligations	0	0	0
Prior service cost (credit) amortization	(565)	(565)	(120)
Actuarial loss (gain) amortization	(1,224)	(798)	(464)
Periodic benefit costs, net	$ 62	$ 727	$ 2,537
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.56%	3.30%	4.20%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	4.13%	3.56%	3.30%
Qualified Pension Benefits | Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 39,677	$ 42,149	$ 37,533
Interest costs	24,007	22,999	30,773
Expected return on assets	(48,159)	(48,235)	(48,055)
Settlement of retiree obligations	0	0	0
Prior service cost (credit) amortization	1,437	1,540	1,606
Actuarial loss (gain) amortization	18,073	22,869	19,016
Periodic benefit costs, net	$ 35,035	$ 41,322	$ 40,873
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.80%	3.60%	4.20%
Expected return on plan assets	5.75%	5.75%	6.00%
Rate of compensation increase	5.08%	5.60%	4.90%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	4.23%	3.80%	3.60%
Rate of compensation increase	5.14%	5.08%	5.60%
Non-Qualified Pension Benefits | Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service costs	$ 548	$ 1,206	$ 1,035
Interest costs	711	843	1,406
Expected return on assets	0	0	0
Settlement of retiree obligations	(112)	(30)	0
Prior service cost (credit) amortization	30	19	228
Actuarial loss (gain) amortization	61	546	692
Periodic benefit costs, net	$ 1,238	$ 2,584	$ 3,361
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.53%	3.28%	4.20%
Rate of compensation increase	5.08%	5.60%	4.90%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]
Discount rate	4.09%	3.53%	3.28%
Rate of compensation increase	5.14%	5.08%	5.60%