Results of Operations for GBW Joint Venture (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Feb. 28, 2017	Feb. 29, 2016	Feb. 28, 2017	Feb. 29, 2016	Feb. 28, 2016	Aug. 31, 2016
Segment Reporting Information [Line Items]
Revenue		$ 566,282	$ 669,079	$ 1,118,596	$ 1,471,468
Earnings (loss) from operations		81,520	92,393	154,180	240,878
Assets		2,155,978		2,155,978			$ 1,835,774
GBW Railcar Services LLC
Segment Reporting Information [Line Items]
Revenue		64,249	97,700		193,682	$ 134,502
Earnings (loss) from operations		(6,901)	$ 3,626		$ 6,034	$ (11,463)
Assets	[1]	$ 227,182		$ 227,182			$ 247,610

[1] Includes goodwill and intangible assets of $91.7 million and $93.4 million as of February 28, 2017 and August 31, 2016.