Fair Value Measurement - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Jul. 31, 2019
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 560,364,000	$ 528,392,000
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 418,500,000	$ 415,600,000
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			238,200,000
Fair value of convertible senior notes			$ 364,000,000.0
Senior convertible notes	0.375%		0.375%