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Fair Value Measurements - Summary of Company's Money Market Funds and Marketable Securities (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total cash equivalents	$ 168,517,000
Total marketable securities	228,962,000	$ 0
Total cash equivalents and marketable securities	397,479,000
Total cash and cash equivalents	168,517,000	186,753,000	$ 127,766,000	$ 8,066,000
Money Market Funds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total cash equivalents	168,517,000
Total cash and cash equivalents		186,753,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total cash equivalents	168,517,000
Total cash equivalents and marketable securities	168,517,000
Total cash and cash equivalents		186,753,000
Fair Value, Inputs, Level 1 [Member] \| Money Market Funds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total cash equivalents	168,517,000
Total cash and cash equivalents		$ 186,753,000
Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	228,962,000
Total cash equivalents and marketable securities	228,962,000
U.S. Government Securities [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	100,031,000
U.S. Government Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	100,031,000
U.S. Corporate Bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	58,452,000
U.S. Corporate Bonds [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	58,452,000
International Corporate Bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	24,190,000
International Corporate Bonds [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	24,190,000
U.S. Commercial Paper [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	30,351,000
U.S. Commercial Paper [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	30,351,000
International Commercial Paper [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	15,938,000
International Commercial Paper [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	$ 15,938,000