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Income Taxes - Roll-forward of Unrecognized Tax Benefits and Associated Accrued Interest and Penalties (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Income Tax Contingency [Line Items]
Opening balance at January 1	$ 16.0
Net change for tax positions of prior periods	0.4
Closing balance at September 30	16.4
Current	0.0
Non-current	16.4	$ 16.0
Interest and Penalties [Member]
Income Tax Contingency [Line Items]
Opening balance at January 1	2.4
Net change for tax positions of prior periods	0.6
Closing balance at September 30	3.0
Current	0.0
Non-current	3.0
Unrecognized Tax Benefits [Member]
Income Tax Contingency [Line Items]
Opening balance at January 1	13.6
Net change for tax positions of prior periods	(0.2)
Closing balance at September 30	13.4
Current	0.0
Non-current	$ 13.4

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Unrecognized tax benefits increases decreases resulting from prior period tax positions.

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Amount recognized for uncertainty in income taxes classified as current.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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Amount of unrecognized tax benefits.

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