Cash-Based Compensation Plans - Narrative (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation Related Costs [Abstract]
Cash based compensation expense	$ 289.8	$ 250.7	$ 233.0

Amount of commitment made to pay deferred cash remuneration.

No definition available.

Name:	us-gaap_DeferredCompensationArrangementWithIndividualCashAwardGrantedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant