Benefit Plans - Changes in Benefit Obligation and Plan (Details) (USD $)	12 Months Ended
	Aug. 31, 2013	Aug. 31, 2012	Aug. 31, 2011
Qualified Pension Benefits
Change in benefit obligation: [Rollforward]
Benefit obligation at beginning of period	$ 671,066,000	$ 501,053,000
Service cost	31,387,000	26,010,000	25,232,000
Interest cost	25,445,000	24,119,000	22,257,000
Transfers in from Agriliance Employees Retirement Plan
Actuarial loss (gain)	12,819,000	982,000
Defined benefit plan, benefit obligation, period increase (decrease)	(64,483,000)	62,755,000
Plan amendments
Defined benefit plan, gross prescription drug subsidy receipts received
Benefits Paid	(34,950,000)	(28,351,000)
Benefit obligation at end of period	641,284,000	671,066,000	501,053,000
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	688,196,000	540,822,000
Actual gain on plan assets	53,582,000	50,515,000
Company contributions	23,800,000	28,000,000
Transfers in from Agriliance Employees Retirement Plan
Benefits Paid	(34,950,000)	(28,351,000)
Fair value of plan assets at end of period	730,628,000	688,196,000	540,822,000
Funded Status of Plan	89,344,000	17,130,000
Assets [Abstract]
Non-current assets	89,930,000	17,695,000
Liabilities [Abstract]
Current liabilities
Non-current liabilities	(586,000)	(565,000)
Ending balance	89,344,000	17,130,000
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Net transition obligation
Prior service cost (credit)	7,794,000	9,392,000
Net loss	253,288,000	331,420,000
Ending balance	261,082,000	340,812,000
Non-Qualified Pension Benefits
Change in benefit obligation: [Rollforward]
Benefit obligation at beginning of period	34,470,000	29,728,000
Service cost	721,000	279,000	1,246,000
Interest cost	1,316,000	1,343,000	1,933,000
Actuarial loss (gain)	3,455,000	2,498,000
Defined benefit plan, benefit obligation, period increase (decrease)	(1,952,000)	1,956,000
Plan amendments
Defined benefit plan, gross prescription drug subsidy receipts received
Benefits Paid	(1,785,000)	(1,334,000)
Benefit obligation at end of period	36,225,000	34,470,000	29,728,000
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	0	0
Actual gain on plan assets
Company contributions	1,785,000	1,334,000
Benefits Paid	(1,785,000)	(1,334,000)
Fair value of plan assets at end of period	0	0	0
Funded Status of Plan	(36,225,000)	(34,470,000)
Assets [Abstract]
Non-current assets
Liabilities [Abstract]
Current liabilities	(3,051,000)	(3,325,000)
Non-current liabilities	(33,174,000)	(31,145,000)
Ending balance	(36,225,000)	(34,470,000)
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Net transition obligation
Prior service cost (credit)	1,088,000	1,316,000
Net loss	10,685,000	10,104,000
Ending balance	11,773,000	11,420,000
Other Benefits
Change in benefit obligation: [Rollforward]
Benefit obligation at beginning of period	64,189,000	56,864,000
Service cost	2,936,000	2,556,000	1,771,000
Interest cost	2,275,000	2,638,000	2,194,000
Actuarial loss (gain)	(5,243,000)	(1,997,000)
Defined benefit plan, benefit obligation, period increase (decrease)	(16,693,000)	6,437,000
Plan amendments		(899,000)
Defined benefit plan, gross prescription drug subsidy receipts received	92,000	625,000
Benefits Paid	(2,014,000)	(2,035,000)
Benefit obligation at end of period	45,542,000	64,189,000	56,864,000
Change in plan assets: [Abstract]
Fair value of plan assets at beginning of period	0	0
Actual gain on plan assets
Company contributions	2,014,000	2,035,000
Benefits Paid	(2,014,000)	(2,035,000)
Fair value of plan assets at end of period	0	0	0
Funded Status of Plan	(45,542,000)	(64,189,000)
Assets [Abstract]
Non-current assets
Liabilities [Abstract]
Current liabilities	(2,919,000)	(3,297,000)
Non-current liabilities	(42,623,000)	(60,892,000)
Ending balance	(45,542,000)	(64,189,000)
Amounts recognized in accumulated other comprehensive loss (pretax): [Abstract]
Net transition obligation		563,000
Prior service cost (credit)	(712,000)	(17,000)
Net loss	(5,415,000)	683,000
Ending balance	$ (6,127,000)	$ 1,229,000