Commitments and Contingencies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) CreditFacility	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Apr. 01, 2015 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Rent expense	$ 15.0	$ 14.3	$ 16.0
Cogent Partners, LP
Commitments and Contingencies Disclosure [Line Items]
Contingent cash obligation				$ 18.9
Letter of Credit
Commitments and Contingencies Disclosure [Line Items]
Number of letters of credit issued to secure office space leases (credit facility) | CreditFacility	5
Amount outstanding on letters of credit	$ 3.9	$ 4.3