Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Rent expense of office facilities and real estate	$ 8.3	$ 8.2	$ 8.1
Rent expense for telecommunications and other equipment	$ 6.0	$ 6.2	$ 5.4