Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($)	3 Months Ended		9 Months Ended
	May 31, 2017	May 31, 2016	May 31, 2017	May 31, 2016
Qualified Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service cost	$ 10,537,000	$ 9,383,000	$ 31,612,000	$ 28,149,000
Interest cost	5,753,000	7,691,000	17,257,000	23,075,000
Expected return on assets	(12,058,000)	(12,013,000)	(36,173,000)	(36,040,000)
Prior service cost (credit) amortization	385,000	402,000	1,155,000	1,205,000
Actuarial (gain) loss amortization	5,708,000	4,765,000	17,123,000	14,294,000
Net periodic benefit cost	10,325,000	10,228,000	30,974,000	30,683,000
Defined Benefit Plan, Contributions by Employer			0
Non-Qualified Pension Benefits
Component of net periodic benefit costs: [Abstract]
Service cost	302,000	258,000	905,000	776,000
Interest cost	210,000	352,000	632,000	1,055,000
Expected return on assets	0	0	0	0
Prior service cost (credit) amortization	4,000	57,000	14,000	171,000
Actuarial (gain) loss amortization	136,000	172,000	409,000	518,000
Net periodic benefit cost	652,000	839,000	1,960,000	2,520,000
Other Benefits
Component of net periodic benefit costs: [Abstract]
Service cost	290,000	353,000	870,000	1,059,000
Interest cost	232,000	427,000	698,000	1,282,000
Expected return on assets	0	0	0	0
Prior service cost (credit) amortization	(141,000)	(30,000)	(424,000)	(90,000)
Actuarial (gain) loss amortization	(199,000)	(116,000)	(598,000)	(348,000)
Net periodic benefit cost	$ 182,000	$ 634,000	$ 546,000	$ 1,903,000