Lease Commitments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Lease Commitments And Contingencies [Line Items]
Operating lease, period of lease, in years	10 years
Rent expenses under operating leases	$ 97.0	$ 95.4	$ 87.6
Computers and Equipment [Member]
Lease Commitments And Contingencies [Line Items]
Operating lease, period of lease, in years	5 years
7 WTC [Member]
Lease Commitments And Contingencies [Line Items]
Operating lease, period of lease, in years	21 years
Renewal options	20 years
Commenced date	Oct. 20, 2006
London [Member]
Lease Commitments And Contingencies [Line Items]
Operating lease, period of lease, in years	17 years 6 months
Renewal options	15 years
Commenced date	Feb. 06, 2008