Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) CreditFacility	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Apr. 01, 2015 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Rent expense	$ 15,000,000	$ 15,000,000	$ 14,300,000
Proceeds from lines of credit	$ 0	69,355,000	$ 105,999,000
Cogent Partners, LP
Commitments and Contingencies Disclosure [Line Items]
Contingent cash obligation				$ 18,900,000
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	$ 3,800,000	3,900,000
Proceeds from lines of credit	$ 0	$ 0