Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Aug. 31, 2016	Aug. 31, 2015	Aug. 31, 2014
Operating Loss Carryforwards [Line Items]
Federal	$ (30,758)	$ 29,348	$ (23,444)
Change in valuation allowance for deferred tax asset	40,600
Operating Loss Carryforwards	676,600
Deferred Tax Assets, Gross [Abstract]
Accrued expenses	87,251	96,270
Postretirment health care and deferred compensation	111,983	89,934
Deferred Tax Assets, Tax Credit Carryforwards	143,252	109,756
Deferred Tax Assets, Operating Loss Carryforwards	155,966	85,860
Other	64,669	68,625
Deferred tax assets valuation	(194,277)	(98,024)
Deferred tax assets	368,844	352,421
Deferred Tax Liabilities, Gross [Abstract]
Pension	26,516	20,732
Investments	109,610	98,291
Major maintenance	4,970	36,135
Property, plant and equipment	679,266	654,057
Deferred Tax Liabilities, Other	33,779	25,836
Deferred Tax Liabilities, Gross	854,141	835,051
Net deferred tax liabilities	485,297	482,630
Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Federal	25,600
NCRA [Member]
Operating Loss Carryforwards [Line Items]
Change in valuation allowance for deferred tax asset	55,600
Operating Loss Carryforwards	$ 133,500	62,200
Other Assets [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent		$ 1,600