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Stock-based Compensation Plans - Allocation of Stock-Based Compensation by Functional Category (Parenthetical) (Detail) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |  |
Accelerated vesting of equity grants | $ 30.8 | $ 30.8

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- Definition Additional share-based compensation cost recognized as a result of an occurrence of an event that accelerates its recognition. + References No definition available. + Details
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