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Income Taxes - Roll-forward of Unrecognized Tax Benefits and Associated Accrued Interest and Penalties (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Dec. 31, 2018 | Jun. 30, 2019
Income Tax Contingency [Line Items] |  |
Opening balance at January 1 |  | $ 14.0
Additions for tax positions of prior periods |  | 0.6
Closing balance at June 30 | $ 14.0 | 14.6
Current |  | 0.0
Non-current | 14.0 | 14.6
Unrecognized Tax Benefits [Member] |  |
Income Tax Contingency [Line Items] |  |
Opening balance at January 1 |  | 13.4
Additions for tax positions of prior periods |  | 0.0
Closing balance at June 30 | 13.4 | 13.4
Current |  | 0.0
Non-current |  | 13.4
Interest and Penalties [Member] |  |
Income Tax Contingency [Line Items] |  |
Opening balance at January 1 |  | 0.6
Additions for tax positions of prior periods |  | 0.6
Closing balance at June 30 | $ 0.6 | 1.2
Current |  | 0.0
Non-current |  | $ 1.2

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