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, 2020	Aug. 31, 2019	Aug. 31, 2018
Change in plan assets: [Abstract]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 46,400
Pension Benefits
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	909,427
Fair value of plan assets at end of period	976,542	$ 909,427
Other Benefits
Change in benefit obligation:
Projected benefit obligation at beginning of period	31,098	29,790
Service cost	1,050	1,053	$ 943
Interest cost	747	1,094	908
Actuarial (gain) loss	(2,286)	(2,596)
Assumption change	1,275	3,398
Plan Amendments	0	0
Settlements	0	0
Benefits paid	(1,568)	(1,641)
Projected benefit obligation at end of period	30,316	31,098	29,790
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	1,568	1,641
Defined Benefit Plan, Plan Assets, Payment for Settlement	0	0
Benefits Paid	(1,568)	(1,641)
Fair value of plan assets at end of period	0	0	0
Funded Status of Plan	(30,316)	(31,098)
ASSETS
Non-current assets	0	0
Liabilities [Abstract]
Current liabilities	(2,090)	(2,040)
Non-current liabilities	(28,226)	(29,058)
Ending balance	(30,316)	(31,098)
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	(2,715)	(3,160)
Net (gain) loss	(15,064)	(15,445)
Ending balance	(17,779)	(18,605)
Qualified Pension Benefits | Pension Benefits
Change in benefit obligation:
Projected benefit obligation at beginning of period	876,696	767,184
Service cost	42,151	38,592	39,677
Interest cost	21,722	28,396	24,007
Actuarial (gain) loss	6,265	(9,606)
Assumption change	40,694	102,441
Plan Amendments	0	18
Settlements	0	615
Benefits paid	(69,526)	(49,714)
Projected benefit obligation at end of period	918,002	876,696	767,184
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	909,427	829,616
Actual gain (loss) on plan assets	90,241	90,139
Defined Benefit Plan, Plan Assets, Contributions by Employer	46,400	40,001
Defined Benefit Plan, Plan Assets, Payment for Settlement	0	(615)
Benefits Paid	(69,526)	(49,714)
Fair value of plan assets at end of period	976,542	909,427	829,616
Funded Status of Plan	58,540	32,731
ASSETS
Non-current assets	58,540	32,731
Liabilities [Abstract]
Current liabilities	0	0
Non-current liabilities	0	0
Ending balance	58,540	32,731
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	938	1,117
Net (gain) loss	225,983	244,164
Ending balance	226,921	245,281
Nonqualified Pension Benefits | Pension Benefits
Change in benefit obligation:
Projected benefit obligation at beginning of period	19,047	20,755
Service cost	405	311	548
Interest cost	429	747	711
Actuarial (gain) loss	1,382	76
Assumption change	775	1,841
Plan Amendments	0	0
Settlements	(2,130)	(3,975)
Benefits paid	(725)	(708)
Projected benefit obligation at end of period	19,183	19,047	20,755
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	2,855	4,683
Defined Benefit Plan, Plan Assets, Payment for Settlement	(2,130)	(3,975)
Benefits Paid	(725)	(708)
Fair value of plan assets at end of period	0	0	$ 0
Funded Status of Plan	(19,183)	(19,047)
ASSETS
Non-current assets	0	0
Liabilities [Abstract]
Current liabilities	(1,660)	(1,580)
Non-current liabilities	(17,523)	(17,467)
Ending balance	(19,183)	(19,047)
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	(502)	(616)
Net (gain) loss	3,813	2,151
Ending balance	$ 3,311	$ 1,535