Benefit Plans - Changes in Benefit Obligation and Plan (Details) - USD ($)	12 Months Ended
	Aug. 31, 2016	Aug. 31, 2015	Aug. 31, 2014
Qualified Pension Benefits
Change in benefit obligation: [Rollforward]
Benefit obligation at beginning of period	$ 730,795,000	$ 720,893,000
Service cost	37,533,000	36,006,000	$ 30,417,000
Interest cost	30,773,000	28,046,000	29,900,000
Actuarial loss (gain)	361,000	20,993,000
Assumption change	57,385,000	(16,297,000)
Plan Amendments	411,000	0
Settlements	0	0
Benefits Paid	(44,509,000)	(58,846,000)
Benefit obligation at end of period	812,749,000	730,795,000	720,893,000
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	796,379,000	822,125,000
Actual gain (loss) on plan assets	88,089,000	(6,065,000)
Company contributions	43,306,000	39,165,000
Defined Benefit Plan, Settlements, Plan Assets	0	0
Benefits Paid	(44,509,000)	(58,846,000)
Fair value of plan assets at end of period	883,265,000	796,379,000	822,125,000
Funded Status of Plan	70,516,000	65,584,000
Assets [Abstract]
Non-current assets	70,594,000	65,927,000
Liabilities [Abstract]
Current liabilities	0	0
Non-current liabilities	(78,000)	(343,000)
Ending balance	70,516,000	65,584,000
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	4,021,000	5,217,000
Net loss	275,146,000	276,450,000
Ending balance	279,167,000	281,667,000
Non-Qualified Pension Benefits
Change in benefit obligation: [Rollforward]
Benefit obligation at beginning of period	33,184,000	37,983,000
Service cost	1,035,000	875,000	860,000
Interest cost	1,406,000	1,414,000	1,660,000
Actuarial loss (gain)	(3,333,000)	393,000
Assumption change	2,679,000	(1,082,000)
Plan Amendments	(1,045,000)	0
Settlements	0	(5,715,000)
Benefits Paid	(1,230,000)	(684,000)
Benefit obligation at end of period	32,696,000	33,184,000	37,983,000
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	1,230,000	6,399,000
Defined Benefit Plan, Settlements, Plan Assets	0	(5,715,000)
Benefits Paid	(1,230,000)	(684,000)
Fair value of plan assets at end of period	0	0	0
Funded Status of Plan	(32,696,000)	(33,184,000)
Assets [Abstract]
Non-current assets	0	0
Liabilities [Abstract]
Current liabilities	(1,880,000)	(1,752,000)
Non-current liabilities	(30,816,000)	(31,432,000)
Ending balance	(32,696,000)	(33,184,000)
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	(641,000)	631,000
Net loss	7,815,000	9,161,000
Ending balance	7,174,000	9,792,000
Other Benefits
Change in benefit obligation: [Rollforward]
Benefit obligation at beginning of period	41,997,000	44,318,000
Service cost	1,412,000	1,513,000	1,729,000
Interest cost	1,709,000	1,489,000	1,918,000
Actuarial loss (gain)	(4,892,000)	1,563,000
Assumption change	2,602,000	(5,136,000)
Plan Amendments	(4,495,000)	0
Settlements	0	0
Benefits Paid	(1,554,000)	(1,750,000)
Benefit obligation at end of period	36,779,000	41,997,000	44,318,000
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	1,554,000	1,750,000
Defined Benefit Plan, Settlements, Plan Assets	0	0
Benefits Paid	(1,554,000)	(1,750,000)
Fair value of plan assets at end of period	0	0	$ 0
Funded Status of Plan	(36,779,000)	(41,997,000)
Assets [Abstract]
Non-current assets	0	0
Liabilities [Abstract]
Current liabilities	(2,490,000)	(2,708,000)
Non-current liabilities	(34,289,000)	(39,289,000)
Ending balance	(36,779,000)	(41,997,000)
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Prior service cost (credit)	(4,847,000)	(472,000)
Net loss	(12,235,000)	(10,409,000)
Ending balance	$ (17,082,000)	$ (10,881,000)