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Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($)
$ in Thousands
 12 Months Ended
Aug. 31, 2019
Aug. 31, 2018
Aug. 31, 2017
Other Benefits      
Component of net periodic benefit costs: [Abstract]      
Service costs $ 1,053 $ 943 $ 1,160
Interest costs 1,094 908 930
Expected return on assets 0 0 0
Settlement of retiree obligations 0 0 0
Prior service cost (credit) amortization (556) (565) (565)
Actuarial loss (gain) amortization (1,627) (1,224) (798)
Periodic benefit costs, net $ (36) $ 62 $ 727
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]      
Discount rate 4.08% 3.56% 3.30%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]      
Discount rate 2.89% 4.13% 3.56%
Qualified Pension Benefits | Pension Benefits      
Component of net periodic benefit costs: [Abstract]      
Service costs $ 38,592 $ 39,677 $ 42,149
Interest costs 28,396 24,007 22,999
Expected return on assets (44,968) (48,159) (48,235)
Settlement of retiree obligations 51 0 0
Prior service cost (credit) amortization 190 1,437 1,540
Actuarial loss (gain) amortization 12,348 18,073 22,869
Periodic benefit costs, net $ 34,609 $ 35,035 $ 41,322
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]      
Discount rate 4.23% 3.80% 3.60%
Expected return on plan assets 5.50% 5.75% 5.75%
Rate of compensation increase 5.14% 5.08% 5.60%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]      
Discount rate 3.06% 4.23% 3.80%
Rate of compensation increase 5.28% 5.14% 5.08%
Non-Qualified Pension Benefits | Pension Benefits      
Component of net periodic benefit costs: [Abstract]      
Service costs $ 311 $ 548 $ 1,206
Interest costs 747 711 843
Expected return on assets 0 0 0
Settlement of retiree obligations 191 (112) (30)
Prior service cost (credit) amortization (75) 30 19
Actuarial loss (gain) amortization 2 61 546
Periodic benefit costs, net $ 1,176 $ 1,238 $ 2,584
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]      
Discount rate 4.09% 3.53% 3.28%
Rate of compensation increase 5.14% 5.08% 5.60%
Weighted-average assumptions to determine the net periodic benefit cost: [Abstract]      
Discount rate 2.70% 4.09% 3.53%
Rate of compensation increase 5.28% 5.14% 5.08%