Stock-based Compensation Plans - Allocation of Stock-Based Compensation by Functional Category (Parenthetical) (Detail) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Accelerated vesting of equity grants	$ 18.2

Additional share-based compensation cost recognized as a result of an occurrence of an event that accelerates its recognition.

No definition available.

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration