Retirement Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
United States
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan	$ 7.5	$ 7.4	$ 6.3
United States | Retirement Savings Contributions
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan	4.9	4.2	3.8
United States | Non Discretionary Matching Contributions
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan	2.6	3.2	2.5
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan	$ 2.6	2.9	2.9
United Kingdom | Employees hired prior to November 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	15.00%
United Kingdom | Employees hired after November 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	12.00%
European Offices
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan	$ 0.3	$ 0.5	$ 0.5
European Offices | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	2.10%
European Offices | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	26.00%