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Other Noncurrent Assets - Additional Information (Detail) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Other Assets [Abstract]        
Operating lease expense $ 4.1      
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