Commitments and Contingencies - Additional Information (Detail) $ in Thousands, £ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2016 USD ($)
Operating Leased Assets [Line Items]
Number of cities in which office spaces leased		47
Lease expiration period description		Through 2026
Number of countries in which office spaces leased	25
Rent expense, costs of equipment leases and maintenance costs	$ 30,800	$ 29,600	$ 32,300
Aggregate minimum future payments on office leases					$ 166,430
Sublease expiration period		Through 2016
Asset retirement obligation		$ 2,300			2,600
Income Tax Examination, Liability (Refund) Adjustment from Settlement with Taxing Authority				£ 3.9	4,800
Office Leases [Member]
Operating Leased Assets [Line Items]
Aggregate minimum future payments on office leases					$ 164,222