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Goodwill - Summary of Goodwill (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Historical accumulated amortization	$ 298.5	$ 298.5

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- Definition

Goodwill historical accumulated amortization.

+ References

No definition available.

+ Details

Name:	iosp_GoodwillHistoricalAccumulatedAmortization
Namespace Prefix:	iosp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- References

No definition available.

+ Details

Name:	us-gaap_GoodwillAndIntangibleAssetsDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration