Leases - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Leases [Line Items]
Operating lease, weighted average remaining lease term	5 years 8 months 19 days
Finance lease, weighted average remaining lease term	3 years 7 months 17 days
Operating lease, weighted average discount rate, percent	5.71%
Finance lease, weighted average discount rate, percent	5.12%
Rental expense for operating leases		$ 16.6	$ 14.8
Sublease income		0.5	0.5
Assets recorded under capital leases, net		8.5	6.6
Assets recorded under capital leases, accumulated amortization		$ 2.9	$ 2.3
Minimum [Member]
Leases [Line Items]
Operating and Financing leases, remaining lease terms	1 year
Maximum [Member]
Leases [Line Items]
Operating and Financing leases, remaining lease terms	12 years