Fair Value Measurement - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Jul. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 513,454,000	$ 513,454,000		$ 193,822,000
Loss on extinguishment of debt	(5,650,000)	(5,650,000)
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 414,700,000	$ 414,700,000		$ 126,600,000
0.375% Convertible Senior Notes due 2024
Summary Of Significant Accounting Policies [Line Items]
Principal amount			$ 287,500,000
Notes, due date		Jul. 15, 2024
Notes, frequency of periodic payment		semi-annually
Total convertible senior notes			230,200,000
Fair value of convertible senior notes			$ 287,900,000
Senior convertible notes	0.375%	0.375%	0.375%