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Income Taxes - Roll-forward of Unrecognized Tax Benefits and Associated Accrued Interest and Penalties (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Income Tax Contingency [Line Items] |  |  |
Opening balance at January 1 | $ 16.4 | $ 14.0 | $ 2.5
Additions for tax positions of prior periods | 1.0 | 2.4 | 12.1
Reductions due to lapsed statute of limitations |  |  | (0.6)
Reductions for tax positions of prior periods | (1.4) |  |
Closing balance at 31 December | 16.0 | 16.4 | 14.0
Current | 0.0 | 0.0 | 0.0
Non-current | 16.0 | 16.4 | 14.0
Interest and Penalties [Member] |  |  |
Income Tax Contingency [Line Items] |  |  |
Opening balance at January 1 | 2.0 | 0.6 | 0.3
Additions for tax positions of prior periods | 0.6 | 1.4 | 0.4
Reductions due to lapsed statute of limitations |  |  | (0.1)
Reductions for tax positions of prior periods | (0.2) |  |
Closing balance at 31 December | 2.4 | 2.0 | 0.6
Current | 0.0 | 0.0 | 0.0
Non-current | 2.4 | 2.0 | 0.6
Unrecognized Tax Benefits [Member] |  |  |
Income Tax Contingency [Line Items] |  |  |
Opening balance at January 1 | 14.4 | 13.4 | 2.2
Additions for tax positions of prior periods | 0.4 | 1.0 | 11.7
Reductions due to lapsed statute of limitations |  |  | (0.5)
Reductions for tax positions of prior periods | (1.2) |  |
Closing balance at 31 December | 13.6 | 14.4 | 13.4
Current | 0.0 | 0.0 | 0.0
Non-current | $ 13.6 | $ 14.4 | $ 13.4

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