Fair Value of Financial Instruments - Assets and Liabilities Subject to Fair Value Measurements on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	$ 214,274	$ 192,538
Short-term investments	164,535	166,993	$ 164,535
Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	3,993	0
Short-term investments	0	699
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	84,081	84,144
US Government and US Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	15,454	16,364
Short-term investments	46,227	42,983
Corporate Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	117,829	122,832
Bank Deposits [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	479	479
Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total financial assets	268,238	268,052
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	3,993
Short-term investments	0	699
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	84,081	84,144
Fair Value, Measurements, Recurring [Member] | US Government and US Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	15,454	16,364
Short-term investments	46,227	42,983
Fair Value, Measurements, Recurring [Member] | Corporate Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	117,829	122,832
Fair Value, Measurements, Recurring [Member] | Bank Deposits [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	479	479
Fair Value, Measurements, Recurring [Member] | Level I [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total financial assets	84,081	84,144
Fair Value, Measurements, Recurring [Member] | Level I [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Short-term investments	0	0
Fair Value, Measurements, Recurring [Member] | Level I [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	84,081	84,144
Fair Value, Measurements, Recurring [Member] | Level I [Member] | US Government and US Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term investments	0	0
Fair Value, Measurements, Recurring [Member] | Level I [Member] | Corporate Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	0	0
Fair Value, Measurements, Recurring [Member] | Level I [Member] | Bank Deposits [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	0	0
Fair Value, Measurements, Recurring [Member] | Level II [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total financial assets	184,157	183,908
Fair Value, Measurements, Recurring [Member] | Level II [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	3,993
Short-term investments	0	699
Fair Value, Measurements, Recurring [Member] | Level II [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring [Member] | Level II [Member] | US Government and US Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	15,454	16,364
Short-term investments	46,227	42,983
Fair Value, Measurements, Recurring [Member] | Level II [Member] | Corporate Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	117,829	122,832
Fair Value, Measurements, Recurring [Member] | Level II [Member] | Bank Deposits [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	479	479
Fair Value, Measurements, Recurring [Member] | Level III [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total financial assets	0	0
Fair Value, Measurements, Recurring [Member] | Level III [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Short-term investments	0	0
Fair Value, Measurements, Recurring [Member] | Level III [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring [Member] | Level III [Member] | US Government and US Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term investments	0	0
Fair Value, Measurements, Recurring [Member] | Level III [Member] | Corporate Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	0	0
Fair Value, Measurements, Recurring [Member] | Level III [Member] | Bank Deposits [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	0	0
Fair Value, Measurements, Recurring [Member] | Forward Foreign Exchange Contracts [Member] | Prepaid Expenses and Other Current Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Prepaid expenses and other current assets - Forward foreign exchange contracts	175	551
Fair Value, Measurements, Recurring [Member] | Forward Foreign Exchange Contracts [Member] | Level I [Member] | Prepaid Expenses and Other Current Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Prepaid expenses and other current assets - Forward foreign exchange contracts	0	0
Fair Value, Measurements, Recurring [Member] | Forward Foreign Exchange Contracts [Member] | Level II [Member] | Prepaid Expenses and Other Current Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Prepaid expenses and other current assets - Forward foreign exchange contracts	175	551
Fair Value, Measurements, Recurring [Member] | Forward Foreign Exchange Contracts [Member] | Level III [Member] | Prepaid Expenses and Other Current Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Prepaid expenses and other current assets - Forward foreign exchange contracts	$ 0	$ 0