Fair Value Measurements - Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	May 31, 2014	Aug. 31, 2013	May 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 544,382	$ 499,890	$ 524,461
Derivative Liability, Fair Value, Gross Liability	412,471	465,066	456,476
Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Inventory, Current, Fair Value Disclosure	0	0	0
Other Assets, Fair Value Disclosure	89,848	114,084	131,901
Total assets	176,669	179,419	190,319
Total liabilities	81,437	65,109	99,880
Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Inventory, Current, Fair Value Disclosure	1,159,581	1,203,383	1,433,639
Other Assets, Fair Value Disclosure	0	0	0
Total assets	1,617,142	1,637,940	1,899,682
Total liabilities	331,034	399,958	356,596
Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Inventory, Current, Fair Value Disclosure	0	0	0
Other Assets, Fair Value Disclosure	0	0	0
Total assets	0	0	0
Total liabilities	130,159	134,134	146,245
Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Inventory, Current, Fair Value Disclosure	1,159,581	1,203,383	1,433,639
Other Assets, Fair Value Disclosure	89,848	114,084	131,901
Total assets	1,793,811	1,817,359	2,090,001
Total liabilities	542,630	599,201	602,721
Foreign Exchange Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	182	7,079	5,043
Derivative Liability, Fair Value, Gross Liability	2,153	5,925	3,883
Foreign Exchange Contract [Member] \| Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	182	6,894	5,043
Derivative Liability, Fair Value, Gross Liability	2,153	5,925	3,883
Foreign Exchange Contract [Member] \| Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	185	0
Derivative Liability, Fair Value, Gross Liability	0	0	0
Foreign Exchange Contract [Member] \| Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	0	0
Derivative Liability, Fair Value, Gross Liability	0	0	0
Foreign Exchange Contract [Member] \| Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	182	7,079	5,043
Derivative Liability, Fair Value, Gross Liability	2,153	5,925	3,883
Commodity and Freight Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	544,198	468,673	510,258
Derivative Liability, Fair Value, Gross Liability	410,148	458,893	452,289
Commodity and Freight Derivatives [Member] \| Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	86,639	58,441	53,375
Derivative Liability, Fair Value, Gross Liability	79,284	59,184	95,997
Commodity and Freight Derivatives [Member] \| Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	457,559	410,233	456,883
Derivative Liability, Fair Value, Gross Liability	330,864	399,710	356,292
Commodity and Freight Derivatives [Member] \| Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	0	0
Derivative Liability, Fair Value, Gross Liability	0	0	0
Commodity and Freight Derivatives [Member] \| Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	544,198	468,674	510,258
Derivative Liability, Fair Value, Gross Liability	410,148	458,894	452,289
Interest Rate Swap [Member] \| Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	0	0
Derivative Liability, Fair Value, Gross Liability	0	0	0
Interest Rate Swap [Member] \| Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	2	24,139	9,160
Derivative Liability, Fair Value, Gross Liability	170	248	304
Interest Rate Swap [Member] \| Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	0	0
Derivative Liability, Fair Value, Gross Liability	0	0	0
Interest Rate Swap [Member] \| Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	2	24,139	9,160
Derivative Liability, Fair Value, Gross Liability	170	248	304
National Cooperative Refinery Association [Member] \| Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued liability for contingent crack spread payments related to purchase of noncontrolling interest	0	0	0
National Cooperative Refinery Association [Member] \| Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued liability for contingent crack spread payments related to purchase of noncontrolling interest	0	0	0
National Cooperative Refinery Association [Member] \| Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued liability for contingent crack spread payments related to purchase of noncontrolling interest	130,159	134,134	146,245
National Cooperative Refinery Association [Member] \| Fair Value, Measurements, Recurring [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued liability for contingent crack spread payments related to purchase of noncontrolling interest		$ 134,134	$ 146,245