Retirement Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
401(k) plan expense	$ 7.4	$ 6.3	$ 7.5
Defined Contribution Plan, employer discretionary matching contribution amount			1.6
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
401(k) plan expense	2.9	$ 2.9	2.6
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]
401(k) plan expense	$ 0.5	$ 0.5	0.2
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%
Retirement Savings Contributions
Defined Benefit Plan Disclosure [Line Items]
401(k) plan expense	$ 4.2	3.8	3.8
Non Discretionary Matching Contributions
Defined Benefit Plan Disclosure [Line Items]
401(k) plan expense	$ 3.2	$ 2.5	$ 2.1