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Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	May 02, 2017	Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Convertible senior notes principal amount		$ 517,500	$ 517,500
2.25% Convertible Senior Notes due 2024
Debt Instrument [Line Items]
Convertible senior notes principal amount	$ 517,500
Convertible senior notes interest percentage	2.25%
Convertible senior notes convertible latest date	May 15, 2024
Convertible senior notes term	7 years
2.25% Convertible Senior Notes due 2024 | Significant Other Observable Inputs (Level 2)
Debt Instrument [Line Items]
Debt instrument fair value		$ 708,700	$ 616,100

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Latest date when the debt instrument can be converted into equity, in CCYY-MM-DD format.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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