Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($)	3 Months Ended		9 Months Ended
	May 31, 2018	May 31, 2017	May 31, 2018	May 31, 2017
Qualified Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service cost	$ 9,920,000	$ 10,537,000	$ 29,758,000	$ 31,612,000
Interest cost	5,997,000	5,753,000	17,988,000	17,257,000
Expected return on assets	(12,044,000)	(12,058,000)	(36,133,000)	(36,173,000)
Prior service cost (credit) amortization	360,000	385,000	1,078,000	1,155,000
Actuarial (gain) loss amortization	4,905,000	5,708,000	16,304,000	17,123,000
Net periodic benefit cost	9,138,000	10,325,000	28,995,000	30,974,000
Defined Benefit Plan, Plan Assets, Contributions by Employer			0
Non-Qualified Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service cost	137,000	302,000	411,000	905,000
Interest cost	177,000	210,000	533,000	632,000
Expected return on assets	0	0	0	0
Prior service cost (credit) amortization	7,000	4,000	23,000	14,000
Actuarial (gain) loss amortization	16,000	136,000	46,000	409,000
Net periodic benefit cost	337,000	652,000	1,013,000	1,960,000
Other Benefits
Component of net periodic benefit costs: [Abstract]
Service cost	236,000	290,000	707,000	870,000
Interest cost	227,000	232,000	681,000	698,000
Expected return on assets	0	0	0	0
Prior service cost (credit) amortization	(142,000)	(141,000)	(424,000)	(424,000)
Actuarial (gain) loss amortization	(306,000)	(199,000)	(918,000)	(598,000)
Net periodic benefit cost	$ 15,000	$ 182,000	$ 46,000	$ 546,000