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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income Tax Contingency [Line Items] |  |  |
Opening balance at January 1 | $ 2.5 | $ 2.3 | $ 3.9
Reductions for tax positions of prior periods | 0.0 | 0.0 | 0.7
Additions for tax positions of prior periods | 12.1 | 0.7 | 0.0
Additions for current year tax positions | 0.0 | 0.0 | 0.0
Reductions due to lapsed statute of limitations | (0.6) | (0.5) | (0.9)
Closing balance at 31 December | 14.0 | 2.5 | 2.3
Current | 0.0 | 0.0 | 0.0
Non-current | 14.0 | 2.5 | 2.3
Interest and Penalties [Member] |  |  |
Income Tax Contingency [Line Items] |  |  |
Opening balance at January 1 | 0.3 | 0.1 | 0.3
Reductions for tax positions of prior periods | 0.0 | 0.0 | 0.1
Additions for tax positions of prior periods | 0.4 | 0.2 | 0.0
Additions for current year tax positions | 0.0 | 0.0 | 0.0
Reductions due to lapsed statute of limitations | (0.1) | 0.0 | (0.1)
Closing balance at 31 December | 0.6 | 0.3 | 0.1
Current | 0.0 | 0.0 | 0.0
Non-current | 0.6 | 0.3 | 0.1
Unrecognized Tax Benefits [Member] |  |  |
Income Tax Contingency [Line Items] |  |  |
Opening balance at January 1 | 2.2 | 2.2 | 3.6
Reductions for tax positions of prior periods | 0.0 | 0.0 | 0.6
Additions for tax positions of prior periods | 11.7 | 0.5 | 0.0
Additions for current year tax positions | 0.0 | 0.0 | 0.0
Reductions due to lapsed statute of limitations | (0.5) | (0.5) | (0.8)
Closing balance at 31 December | 13.4 | 2.2 | 2.2
Current | 0.0 | 0.0 | 0.0
Non-current | $ 13.4 | $ 2.2 | $ 2.2

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