Results of Operations for GBW Joint Venture (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Aug. 31, 2017	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2016	May 31, 2016	Feb. 28, 2016	Nov. 30, 2015	Aug. 31, 2017		Aug. 31, 2016		Aug. 31, 2015
Segment Reporting Information [Line Items]
Revenue		$ 611,407	$ 439,161	$ 566,282	$ 552,314	$ 595,190	$ 612,866	$ 669,079	$ 802,389	$ 2,169,164	[1]	$ 2,679,524	[1]	$ 2,605,278	[1]
Earnings (loss) from operations		57,780	$ 48,472	$ 81,520	$ 72,660	83,545	$ 84,129	$ 92,393	$ 148,485	260,432		408,552		386,892
Assets		2,397,705				1,835,774				2,397,705		1,835,774		1,787,452
Capital expenditures										86,065		139,013		105,989
GBW Railcar Services LLC
Segment Reporting Information [Line Items]
Revenue										253,436		373,490		349,849
Earnings (loss) from operations										(32,454)		8,558		(1,160)
Assets	[2]	$ 206,009				$ 247,610				206,009		247,610		239,871
Depreciation and amortization										9,023		7,676		4,590
Capital expenditures										$ 8,030		$ 16,110		$ 26,396

[1] Revenue is presented on the basis of geographic location of customers.

[2] Includes goodwill and intangible assets of $78.8 million, $93.4 million and $96.9 million as of August 31, 2017, 2016 and 2015, respectively. In 2017, GBW recorded a pre-tax goodwill impairment loss of $11.2 million which reduced the goodwill balance to $41.5 million.