Segment Information - Results of Operations for GBW Joint Venture (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Feb. 28, 2018	Feb. 28, 2017	Feb. 28, 2018	Feb. 28, 2017	Aug. 31, 2017
Segment Reporting Information [Line Items]
Revenue		$ 629,336	$ 566,282	$ 1,188,871	$ 1,118,596
Loss from operations		60,870	81,520	122,312	154,180
Assets		2,432,055		2,432,055		$ 2,397,705
GBW Railcar Services LLC
Segment Reporting Information [Line Items]
Revenue		62,685	64,249	120,685	134,502
Loss from operations		(5,460)	$ (6,901)	(11,204)	$ (11,463)
Assets	[1]	$ 208,522		$ 208,522		$ 206,009

[1] Includes goodwill and intangible assets of $77.5 million and $78.8 million as of February 28, 2018 and August 31, 2017.