Results of Operations for GBW Joint Venture (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		May 31, 2016	May 31, 2015	May 31, 2016	May 31, 2015	Aug. 31, 2015
Segment Reporting Information [Line Items]
Revenue		$ 612,866	$ 714,606	$ 2,084,334	$ 1,839,813
Earnings (loss) from operations		84,129	104,411	325,007	251,365
Assets		1,785,884		1,785,884		$ 1,790,512
GBW Railcar Services Holdings, LLC
Segment Reporting Information [Line Items]
Revenue		95,699	88,831	289,381	254,688
Earnings (loss) from operations		3,030	227	9,065	(1,507)
Assets	[1]	$ 255,400	$ 239,871	$ 255,400	$ 239,871

[1] Includes goodwill and intangible assets of $94.4 million and $96.9 million as of May 31, 2016 and August 31, 2015.