Benefit Plans - Changes in Benefit Obligation and Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2015
Qualified Pension Benefits
Change in benefit obligation: [Rollforward]
Benefit obligation at beginning of period	$ 812,749	$ 730,795
Service cost	42,149	37,533	$ 36,006
Interest cost	22,999	30,773	28,046
Actuarial (gain) loss	(10,054)	361
Assumption change	(17,750)	57,385
Plan Amendments	0	411
Settlements	0	0
Benefits paid	(43,919)	(44,509)
Benefit obligation at end of period	806,174	812,749	730,795
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	883,265	796,379
Actual gain (loss) on plan assets	36,474	88,089
Company contributions	0	43,306
Defined Benefit Plan, Plan Assets, Payment for Settlement	0	0
Benefits Paid	(43,919)	(44,509)
Fair value of plan assets at end of period	875,820	883,265	796,379
Funded Status of Plan	69,646	70,516
Assets [Abstract]
Non-current assets	70,019	70,594
Liabilities [Abstract]
Current liabilities	0	0
Non-current liabilities	(373)	(78)
Ending balance	69,646	70,516
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	2,481	4,021
Net (gain) loss	236,232	275,146
Ending balance	238,713	279,167
Non-Qualified Pension Benefits
Change in benefit obligation: [Rollforward]
Benefit obligation at beginning of period	32,696	33,184
Service cost	1,206	1,035	875
Interest cost	843	1,406	1,414
Actuarial (gain) loss	(5,692)	(3,333)
Assumption change	(655)	2,679
Plan Amendments	0	(1,045)
Settlements	2,131	0
Benefits paid	(668)	(1,230)
Benefit obligation at end of period	25,599	32,696	33,184
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	0	0
Actual gain (loss) on plan assets	0	0
Company contributions	2,799	1,230
Defined Benefit Plan, Plan Assets, Payment for Settlement	(2,131)	0
Benefits Paid	(668)	(1,230)
Fair value of plan assets at end of period	0	0	0
Funded Status of Plan	(25,599)	(32,696)
Assets [Abstract]
Non-current assets	0	0
Liabilities [Abstract]
Current liabilities	(2,270)	(1,880)
Non-current liabilities	(23,329)	(30,816)
Ending balance	(25,599)	(32,696)
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	363	(641)
Net (gain) loss	(70)	7,815
Ending balance	293	7,174
Other Benefits
Change in benefit obligation: [Rollforward]
Benefit obligation at beginning of period	36,779	41,997
Service cost	1,160	1,412	1,513
Interest cost	930	1,709	1,489
Actuarial (gain) loss	(4,650)	(4,892)
Assumption change	(775)	2,602
Plan Amendments	0	(4,495)
Settlements	0	0
Benefits paid	(1,608)	(1,554)
Benefit obligation at end of period	31,836	36,779	41,997
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	0	0
Actual gain (loss) on plan assets	0	0
Company contributions	1,608	1,554
Defined Benefit Plan, Plan Assets, Payment for Settlement	0	0
Benefits Paid	(1,608)	(1,554)
Fair value of plan assets at end of period	0	0	$ 0
Funded Status of Plan	(31,836)	(36,779)
Assets [Abstract]
Non-current assets	0	0
Liabilities [Abstract]
Current liabilities	(2,140)	(2,490)
Non-current liabilities	(29,696)	(34,289)
Ending balance	(31,836)	(36,779)
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	(4,281)	(4,847)
Net (gain) loss	(16,864)	(12,235)
Ending balance	$ (21,145)	$ (17,082)