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Commitments and Contingencies (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]        
Rent expense of office facilities and real estate $ 2.2 $ 2.1 $ 6.7 $ 6.1
Rent expense for telecommunications and other equipment $ 1.8 $ 1.5 $ 4.9 $ 4.5