Leases - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Lessee Lease Description [Line Items]
Operating lease, Percentage of office facilities	96.00%
Operating lease, option to extend	Our leases have remaining lease terms of 1 year to 11 years. These leases generally contain renewal options for periods ranging from two to five years. Because the Company is not reasonably certain to exercise these renewal options, the options are not considered in determining the lease term, and associated potential option payments are excluded from lease payments.
Lessee, Operating Lease, Existence of Option to Extend [true false]	true
Operating lease expense			$ 38.0	$ 38.4	$ 37.0
Operating lease payments to related parties	$ 0.6	$ 0.9
Minimum [Member]
Lessee Lease Description [Line Items]
Operating lease, Remaining lease term	1 year
Operating lease, renewal term	2 years
Maximum [Member]
Lessee Lease Description [Line Items]
Operating lease, Remaining lease term	11 years
Operating lease, renewal term	5 years