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Revenue - Schedule of Changes in Deferred Revenue (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Revenue From Contract With Customer [Abstract]    
Deferred revenue, beginning of period $ 23.2 $ 16.5
New billings 63.6 44.9
Revenue recognized (63.8) (41.7)
Deferred revenue, end of period $ 23.0 $ 19.7