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Income Taxes - Roll-forward of Unrecognized Tax Benefits and Associated Accrued Interest and Penalties (Detail) - USD ($) $ in Millions | 6 Months Ended |
--- | --- | ---
Jun. 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 | 0.3 |
Closing balance at 30 June | 16.3 |
Current | 0.0 |
Non-current | 16.3 | $ 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 | 0.4 |
Closing balance at 30 June | 2.8 |
Current | 0.0 |
Non-current | 2.8 |
Unrecognized Tax Benefits [Member] |  |
Income Tax Contingency [Line Items] |  |
Opening balance at January 1 | 13.6 |
Net change for tax positions of prior periods | (0.1) |
Closing balance at 30 June | 13.5 |
Current | 0.0 |
Non-current | $ 13.5 |

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