Segment Information - Results of Operations for GBW Joint Venture (Parenthetical) (Detail) - USD ($) $ in Millions	Feb. 28, 2018	Aug. 31, 2017
GBW Railcar Services LLC
Segment Reporting Information [Line Items]
Goodwill and intangible assets	$ 77.5	$ 78.8

Carrying amount of finite-lived intangible assets, indefinite-lived intangible assets and goodwill. Goodwill is an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. Intangible assets are assets, not including financial assets, lacking physical substance.

No definition available.

Name:	us-gaap_IntangibleAssetsNetIncludingGoodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant