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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions |  | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 01, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Jun. 30, 2018
Income Tax Contingency [Line Items] |  |  |  |  |  |
Income tax expense (benefit) |  | $ (28.4) | $ 6.2 | $ (22.8) | $ 11.4 |
Income tax expense benefit excluding change in enacted tax rate |  | $ 4.2 |  | $ 9.8 |  |
Approximate rate on pre-tax income |  |  | 51.20% |  | 70.80% |
Effective income tax rate continuing operations excluding change in enacted tax rate |  | 113.50% |  | 10.70% |  |
Tax Act [Member] |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Income tax (benefit) due to revaluing of deferred tax liabilities |  | $ (32.6) |  |  |  |
U.S. federal statutory rate |  |  |  | 35.00% |  |
Tax Act [Member] | Scenario, Forecast |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
Blended Corporate Tax Rate |  |  |  |  |  | 28.00%
Tax Act [Member] | Subsequent Event [Member] |  |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |  |
U.S. federal statutory rate | 21.00% |  |  |  |  |

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