Other Noncurrent Assets - 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
Other Assets Non Current [Line Items]
Operating lease expense	$ 2.1
Weighted average remaining lease term	13 years
Weighted average discount rate associated with leases	4.50%
Net operating lease rent expense			$ 14.8	$ 16.3	$ 14.3
Infrastructure reimbursement	$ 1.1	$ 3.8
City Of Henderson
Other Assets Non Current [Line Items]
Infrastructure reimbursement	$ 1.1	$ 3.8