Employee Benefit Plans (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
United States
Defined Contribution Plan Disclosure [Line Items]
Employer contributions to plan	$ 15.2	$ 11.6	$ 11.4
United Kingdom
Defined Contribution Plan Disclosure [Line Items]
Employer contributions to plan	$ 7.7	$ 6.4	$ 6.3

Amount of cost for defined contribution plan.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

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