Commitments and Contingencies (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Rent expense of office facilities and real estate	$ 2.2	$ 1.8	$ 4.5	$ 4.0
Rent expense for telecommunications and other equipment	$ 1.7	$ 1.6	$ 3.1	$ 3.0