Restatement of Previously Issued Consolidated Financial Statements - Summary of Impacts of the Restatement Adjustments on Previously Reported Consolidated Statements of Changes in Equities (Details) - USD ($) $ in Thousands	Aug. 31, 2018	May 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2017	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2016	Aug. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	$ 8,165,028	$ 7,920,836	$ 7,866,357	$ 7,742,948	$ 7,705,640	$ 7,680,371	$ 7,808,580	$ 7,819,516	$ 7,759,159	$ 7,551,439
As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity		8,162,533	8,061,960	7,937,775	7,905,825	7,858,956	7,961,594	7,931,280	7,866,250	7,669,411
Cumulative restatement adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity		(241,697)	(195,603)	(194,827)	(200,185)	(178,585)	(153,014)	(111,764)	(107,091)	(117,972)
Capital Equity Certificates
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	3,837,580				3,906,426				3,918,711	3,793,897
Capital Equity Certificates | As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					3,906,426				3,932,513	3,793,897
Capital Equity Certificates | Cumulative restatement adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					0				(13,802)	0
Nonpatronage Equity Certificates
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	29,498				29,836				22,894	23,057
Nonpatronage Equity Certificates | As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					29,836				22,894	23,057
Nonpatronage Equity Certificates | Cumulative restatement adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					0				0	0
Nonqualified Equity Certificates
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	742,378				405,387				281,767	282,928
Nonqualified Equity Certificates | As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					405,387				281,767	282,928
Nonqualified Equity Certificates | Cumulative restatement adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					0				0	0
Preferred Stock
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	2,264,038				2,264,038				2,244,132	2,167,540
Preferred Stock | As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					2,264,038				2,244,132	2,167,540
Preferred Stock | Cumulative restatement adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					0				0	0
Accumulated Other Comprehensive Loss
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	(199,915)				(180,360)				(211,530)	(212,837)
Accumulated Other Comprehensive Loss | As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					(183,670)				(211,726)	(214,207)
Accumulated Other Comprehensive Loss | Cumulative restatement adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					3,310				196	1,370
Capital Reserves
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	1,482,003				1,267,808				1,488,999	1,485,433
Capital Reserves | As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					1,471,217				1,582,380	1,604,670
Capital Reserves | Cumulative restatement adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					(203,409)				(93,381)	(119,237)
Noncontrolling Interests
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	$ 9,446				12,505				14,186	11,421
Noncontrolling Interests | As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					12,591				14,290	11,526
Noncontrolling Interests | Cumulative restatement adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity					(86)				(104)	(105)
Freight Derivatives and Related Misstatements | Cumulative restatement adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity		$ (209,200)	$ 185,900	$ (163,200)	$ (162,400)	$ 136,800	$ (114,700)	$ (114,000)	$ (115,700)	$ (104,600)