Fair Value Measurements - Summary of Company's Money Market Funds and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 306,235	$ 168,517
Total marketable securities	212,613	228,962
Total cash equivalents and marketable securities	518,848	397,479
Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	306,235	168,517
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	306,235	168,517
Total cash equivalents and marketable securities	306,235	168,517
Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	306,235	168,517
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	212,613	228,962
Total cash equivalents and marketable securities	212,613	228,962
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	49,606	100,031
U.S. Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	49,606	100,031
U.S. Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	48,959	58,452
U.S. Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	48,959	58,452
International Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities		24,190
International Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities		24,190
U.S. Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	65,583	30,351
U.S. Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	65,583	30,351
International Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	48,465	15,938
International Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	$ 48,465	$ 15,938