Investments in Unconsolidated Affiliates - Schedule of Summarized Financial Data (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
Schedule of Equity Method Investments [Line Items]
Total assets		$ 2,465,464				$ 2,397,705				$ 2,465,464		$ 2,397,705		$ 1,835,774
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]
Margin		105,887	108,507	105,347	89,314	99,922	89,701	118,925	112,751	409,055		421,299		551,437
Net income (loss)		30,935	$ 32,951	$ 61,642	$ 26,253	23,746	$ 32,823	$ 34,536	$ 24,962	151,781		116,067		183,213
GBW Railcar Services LLC
Schedule of Equity Method Investments [Line Items]
Current assets		8,531				81,860				8,531		81,860
Total assets		8,531				206,009				8,531		206,009		247,610
Current liabilities		23,283				33,033				23,283		33,033
Total liabilities		23,283				111,384				23,283		111,384
Revenue										238,033		253,436		373,490
Margin										(6,047)		(4,058)		33,929
Net income (loss)	[2]									(51,679)		(36,947)		4,006
Greenbrier-Maxion
Schedule of Equity Method Investments [Line Items]
Current assets		41,619				48,012				41,619		48,012
Total assets		61,034				71,455				61,034		71,455
Current liabilities		38,027				38,055				38,027		38,055
Total liabilities		41,539				42,197				41,539		42,197
Revenue										187,664		228,510		168,465
Margin										10,086		24,372		14,245
Net income (loss)										(3,006)		1,378		(4,051)
Amsted-Maxion Cruzeiro
Schedule of Equity Method Investments [Line Items]
Current assets		21,463				23,777				21,463		23,777
Total assets		111,589				142,583				111,589		142,583
Current liabilities		27,981				28,084				27,981		28,084
Total liabilities		83,407				94,846				83,407		94,846
Revenue										96,490		90,114		87,833
Margin										8,001		5,983		8,256
Net income (loss)										(9,590)		(20,114)		(12,640)
Other Unconsolidated Affiliates
Schedule of Equity Method Investments [Line Items]
Current assets		32,168				16,996				32,168		16,996
Total assets		239,535				283,895				239,535		283,895
Current liabilities		3,647				3,003				3,647		3,003
Total liabilities		$ 52,852				$ 90,064				52,852		90,064
Revenue										25,549		39,161		75,851
Margin										11,360		8,015		11,087
Net income (loss)										$ 6,988		$ 5,202		$ 6,051

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

[2] In 2018 and 2017, GBW recorded a pre-tax goodwill impairment loss of $26.4 million and $11.2 million, respectively, which reduced the goodwill balance to $15.1 million at the time of the dissolution.