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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Jun. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2017
Income Tax Disclosure [Line Items] |  |  |  |  |  |  |  |
U.S. federal corporate tax rate |  |  |  | 35.00% |  |  |  |
Provisional tax expense | $ 46.5 |  |  | $ 46.5 |  |  |  |
Discrete tax expense |  |  |  | 54.4 |  |  |  |
Non-discrete tax benefit |  |  |  | $ (7.9) |  |  |  |
Impact of annual effective tax rate |  |  |  | 12.60% |  |  |  |
Discrete tax provision | 5.1 |  |  | $ 5.1 |  |  |  |
Increase in income tax expense, net of foreign tax credits |  |  |  | 49.3 |  |  |  |
Provision for income taxes | 53.3 | $ 6.6 |  | 56.5 | $ 7.6 |  |  |
Gross unrecognized tax benefits | 18.0 |  |  | 18.0 |  |  |  | $ 15.2
Gross unrecognized tax benefits increased during the year |  |  |  | 2.8 |  |  |  |
Interest and penalties accrued related to unrecognized tax benefits | 1.3 |  |  | 1.3 |  |  |  |
Increase in interest and penalties accrued related to unrecognized tax benefits |  |  |  | 0.1 |  |  |  |
Cumulative-effect adjustment related to retained earnings, tax |  |  |  | 8.3 |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |  |  |  |
Estimated increase in unrecognized tax benefit in next twelve months | $ 2.4 |  |  | 2.4 |  |  |  |
Undistributed Foreign Earnings [Member] |  |  |  |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |  |  |  |
Provision for income taxes |  |  |  | $ 0.0 |  |  |  |
Cash Assets [Member] |  |  |  |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |  |  |  |
One-time transition tax rate on post-1986 foreign earnings and profits | 15.50% |  |  | 15.50% |  |  |  |
Other Specified Assets [Member] |  |  |  |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |  |  |  |
One-time transition tax rate on post-1986 foreign earnings and profits | 8.00% |  |  | 8.00% |  |  |  |
Scenario, Forecast [Member] |  |  |  |  |  |  |  |
Income Tax Disclosure [Line Items] |  |  |  |  |  |  |  |
U.S. federal corporate tax rate |  |  | 21.00% |  |  | 21.00% | 28.17% |

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