Fair Value Measurements - Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | $ 45,364 | $ 38,941 |
Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 49,996 | 49,926 |
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 19,605 | 25,067 |
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 30,391 | 24,859 |
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 0 | 0 |
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | 4,632 | 10,085 |
Money Market Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | 4,632 | 10,085 |
Money Market Funds [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | 0 | 0 |
Money Market Funds [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | 0 | 0 |
U.S. Government Agencies [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 14,973 | 14,982 |
U.S. Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 14,973 | 14,982 |
U.S. Government Agencies [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 14,973 | 14,982 |
U.S. Government Agencies [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
U.S. Government Agencies [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Commercial Paper [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 6,295 | 7,178 |
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 6,295 | 8,078 |
Commercial Paper [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Commercial Paper [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 6,295 | 8,078 |
Commercial Paper [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Corporate Bonds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 17,686 | 10,314 |
Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 17,686 | 10,314 |
Corporate Bonds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Corporate Bonds [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 17,686 | 10,314 |
Corporate Bonds [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Asset-backed Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 6,410 | 6,467 |
Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 6,410 | 6,467 |
Asset-backed Securities [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Asset-backed Securities [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 6,410 | 6,467 |
Asset-backed Securities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | $ 0 | $ 0 |