Segment Information - Results of Operations for GBW Joint Venture (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Aug. 31, 2018	May 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2017	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2018		Aug. 31, 2017		Aug. 31, 2016
Segment Reporting Information [Line Items]
Revenue	$ 689,206	$ 641,387	$ 629,336	$ 559,535	$ 611,407	$ 439,161	$ 566,282	$ 552,314	$ 2,519,464	[1]	$ 2,169,164	[1]	$ 2,679,524	[1]
Earnings (loss) from operations	59,134	$ 71,539	$ 60,870	$ 61,442	57,780	$ 48,472	$ 81,520	$ 72,660	252,985		260,432		408,552
Assets	2,465,464				2,397,705				2,465,464		2,397,705		1,835,774
Capital expenditures									176,848		86,065		139,013
GBW Railcar Services LLC
Segment Reporting Information [Line Items]
Revenue									238,033		253,436		373,490
Earnings (loss) from operations									(46,783)		(32,454)		8,558
Assets	$ 8,531				$ 206,009				8,531		206,009		247,610
Depreciation and amortization									8,932		9,023		7,676
Capital expenditures									$ 8,514		$ 8,030		$ 16,110

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