Fair Value Measurement - Additional Information (Detail) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	May 24, 2016
Summary Of Significant Accounting Policies [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 196,135,000	$ 193,822,000
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash and Cash Equivalents, at Carrying Value	122,300,000	126,600,000
2.125% Convertible Senior Notes due 2021
Summary Of Significant Accounting Policies [Line Items]
Principal amount			$ 17,000	$ 11,000	$ 115,000,000
Total convertible senior notes	104,595,000	$ 103,488,000
Fair value of convertible senior notes	$ 214,700,000