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Leases - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Lessee Lease Description [Line Items]        
Operating lease payment $ 2.1      
Operating lease, expense $ 2.1      
Operating lease, weighted average remaining lease term 13 years      
Operating lease, weighted average discount rate 4.50%      
Total lease obligations $ 32.4      
Aggregate future minimum rental commitments $ 45.4 $ 42.5    
Non-cancellable Operating Leases        
Lessee Lease Description [Line Items]        
Operating lease, expense   15.0 $ 16.0 $ 14.0
Aggregate future minimum rental commitments   42.5    
Leverkusen Facility Lease        
Lessee Lease Description [Line Items]        
Agreements expiry period 2050      
Current Tio2 production capacity in Leverkusen facility 33.00%      
Total lease obligations $ 7.1      
Leverkusen Facility Lease | Non-cancellable Operating Leases        
Lessee Lease Description [Line Items]        
Aggregate future minimum rental commitments   $ 17.0