Quarterly Financial Information - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 338.0	$ 266.7	$ 242.5	$ 184.4	$ 221.0	$ 184.6	$ 170.6	$ 142.3	$ 1,031.6	$ 718.5	$ 573.3
Gross profit	222.8	168.6	153.6	118.9	153.5	127.0	117.5	94.1	663.9	492.1	378.4
Total operating expenses	387.4	154.7	158.5	149.6	141.5	127.5	131.1	134.5	850.2	534.6	442.3
Net income (loss)	$ (179.7)	$ 46.6	$ 30.2	$ (24.2)	$ (9.4)	$ (2.0)	$ 2.9	$ (41.7)	$ (127.1)	$ (50.2)	$ (65.6)
Basic net income (loss) per share	$ (2.03)	$ 0.53	$ 0.34	$ (0.28)	$ (0.11)	$ (0.02)	$ 0.03	$ (0.49)
Diluted net income (loss) per share	$ (2.03)	$ 0.52	$ 0.34	$ (0.28)	$ (0.11)	$ (0.02)	$ 0.03	$ (0.49)