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Summary of Significant Accounting Policies - Income Taxes (Detail) $ in Millions | 9 Months Ended
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Sep. 30, 2017 USD ($)
Income Taxes [Line Items] |
Total amount of unrecognized tax benefits relating to uncertain tax positions | $ 31.8
Amount of unrecognized tax benefits change from year end relating to uncertain tax positions | (3.4)
Decrease in unrecognized tax benefits resulting from expiration of statute of limitations periods | 6.2
Maximum |
Income Taxes [Line Items] |
Reasonably possible reduction to the balance of unrecognized tax benefits in succeeding 12 months | $ 13.7

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