Accrued Compensation and Benefits - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Defined benefit plan obligation	$ 73.4	$ 61.9
Defined benefit plan fair value of the plans' assets	56.3	46.3
Defined benefit plan liability	17.1	15.6
Net periodic benefit cost	$ 4.2	$ 2.5	$ 2.8