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Summary of Significant Accounting Policies - Income Taxes (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2018 | Dec. 31, 2017
Income Taxes [Line Items] |  |  |
Total amount of unrecognized tax benefits relating to uncertain tax positions | $ 30.9 | $ 30.9 |
Decrease in the total amount of unrecognized tax benefits relating to uncertain tax positions |  | (5.3) |
Tax Act, one-time net tax benefits | 5.0 |  | $ 70.1
Tax Act, benefits associated with re-measurement of deferred tax assets and liabilities |  |  | 85.3
Tax Act, transition tax expense on post-1986 earnings and profits of certain foreign subsidiaries |  |  | 15.2
Tax Act, benefit associated with entity restructuring and recapitalization |  |  | 26.7
Tax Act, expense associated with acceleration of certain deductions |  |  | $ 8.5
Tax Act, decrease in benefit associated with entity restructuring and recapitalization | 1.8 |  |
Maximum |  |  |
Income Taxes [Line Items] |  |  |
Reasonably possible reduction to the balance of unrecognized tax benefits in succeeding 12 months | $ 7.7 | $ 7.7 |

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