Benefit Plans - Financial Information on Changes in Benefit Obligation, Plan Assets Funded and Balance Sheets Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Aug. 31, 2019	Aug. 31, 2018	Aug. 31, 2017
Change in plan assets: [Abstract]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 40,000
Pension Benefits
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	829,616
Fair value of plan assets at end of period	909,427	$ 829,616
Other Benefits
Change in benefit obligation:
Projected benefit obligation at beginning of period	29,790	31,836
Service cost	1,053	943	$ 1,160
Interest cost	1,094	908	930
Actuarial (gain) loss	(2,596)	(623)
Assumption change	3,398	(1,612)
Plan Amendments	0	0
Settlements	0	0
Benefits paid	(1,641)	(1,662)
Projected benefit obligation at end of period	31,098	29,790	31,836
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	0	0
Actual gain (loss) on plan assets	0	0
Defined Benefit Plan, Plan Assets, Contributions by Employer	1,641	1,662
Defined Benefit Plan, Plan Assets, Payment for Settlement	0	0
Benefits Paid	(1,641)	(1,662)
Fair value of plan assets at end of period	0	0	0
Funded Status of Plan	(31,098)	(29,790)
ASSETS
Non-current assets	0	0
Liabilities [Abstract]
Current liabilities	(2,040)	(2,040)
Non-current liabilities	(29,058)	(27,750)
Ending balance	(31,098)	(29,790)
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	(3,160)	(3,716)
Net (gain) loss	(15,445)	(17,875)
Ending balance	(18,605)	(21,591)
Qualified Pension Benefits | Pension Benefits
Change in benefit obligation:
Projected benefit obligation at beginning of period	767,184	806,174
Service cost	38,592	39,677	42,149
Interest cost	28,396	24,007	22,999
Actuarial (gain) loss	(9,606)	3,146
Assumption change	102,441	(36,515)
Plan Amendments	18	244
Settlements	(615)	0
Benefits paid	(49,714)	(69,549)
Projected benefit obligation at end of period	876,696	767,184	806,174
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	829,616	875,820
Actual gain (loss) on plan assets	90,139	23,345
Defined Benefit Plan, Plan Assets, Contributions by Employer	40,001	0
Defined Benefit Plan, Plan Assets, Payment for Settlement	(615)	0
Benefits Paid	(49,714)	(69,549)
Fair value of plan assets at end of period	909,427	829,616	875,820
Funded Status of Plan	32,731	62,432
ASSETS
Non-current assets	32,731	62,432
Liabilities [Abstract]
Current liabilities	0	0
Non-current liabilities	0	0
Ending balance	32,731	62,432
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	1,117	1,288
Net (gain) loss	244,164	209,606
Ending balance	245,281	210,894
Non-Qualified Pension Benefits | Pension Benefits
Change in benefit obligation:
Projected benefit obligation at beginning of period	20,755	25,599
Service cost	311	548	1,206
Interest cost	747	711	843
Actuarial (gain) loss	76	205
Assumption change	1,841	(783)
Plan Amendments	0	0
Settlements	(3,975)	(4,824)
Benefits paid	(708)	(701)
Projected benefit obligation at end of period	19,047	20,755	25,599
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	0	0
Actual gain (loss) on plan assets	0	0
Defined Benefit Plan, Plan Assets, Contributions by Employer	4,683	5,525
Defined Benefit Plan, Plan Assets, Payment for Settlement	(3,975)	(4,824)
Benefits Paid	(708)	(701)
Fair value of plan assets at end of period	0	0	$ 0
Funded Status of Plan	(19,047)	(20,755)
ASSETS
Non-current assets	0	0
Liabilities [Abstract]
Current liabilities	(1,580)	(1,780)
Non-current liabilities	(17,467)	(18,975)
Ending balance	(19,047)	(20,755)
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	(616)	(691)
Net (gain) loss	2,151	427
Ending balance	$ 1,535	$ (264)