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Acquisitions - Summary of Changes to Goodwill Balance (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2019	Mar. 31, 2019
Goodwill [Line Items]
Ending balance	$ 415.9	$ 415.9
Counsyl Inc [Member]
Goodwill [Line Items]
Beginning balance		94.9
Fair value adjustment to equipment		0.7
Intangible adjustment	2.0	2.9
Working capital adjustment		(1.1)
Change in deferred tax liability		0.9
Ending balance	$ 98.3	$ 98.3

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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