Fair Value Of Financial Instruments - Summary of Asset and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands | Jun. 30, 2017 | Dec. 31, 2016 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | $ 296,709 | $ 305,901 |
Fair value measurements, recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 76,707 | 48,136 |
Assets measured at fair value on a recurring basis | 296,709 | 305,901 |
Fair value measurements, recurring | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 62,709 | 48,136 |
Assets measured at fair value on a recurring basis | 62,709 | 48,136 |
Fair value measurements, recurring | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 13,998 | 0 |
Assets measured at fair value on a recurring basis | 234,000 | 257,765 |
Fair value measurements, recurring | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 0 | 0 |
Assets measured at fair value on a recurring basis | 0 | 0 |
Corporate bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 57,262 | 60,676 |
Corporate bonds | Fair value measurements, recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 57,262 | 60,676 |
Corporate bonds | Fair value measurements, recurring | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Corporate bonds | Fair value measurements, recurring | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 57,262 | 60,676 |
Corporate bonds | Fair value measurements, recurring | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Agency bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 7,993 | 28,930 |
Agency bonds | Fair value measurements, recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 7,993 | 28,930 |
Agency bonds | Fair value measurements, recurring | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Agency bonds | Fair value measurements, recurring | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 7,993 | 28,930 |
Agency bonds | Fair value measurements, recurring | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
U.S. treasury securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 152,778 | 157,686 |
U.S. treasury securities | Fair value measurements, recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 138,780 | 157,686 |
U.S. treasury securities | Fair value measurements, recurring | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
U.S. treasury securities | Fair value measurements, recurring | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 138,780 | 157,686 |
U.S. treasury securities | Fair value measurements, recurring | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Commercial paper | Fair value measurements, recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 15,967 | 10,473 |
Commercial paper | Fair value measurements, recurring | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Commercial paper | Fair value measurements, recurring | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 15,967 | 10,473 |
Commercial paper | Fair value measurements, recurring | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | $ 0 | $ 0 |