Financial Instruments (Detail) - Financial Assets Measured At Fair Value On A Recurring Basis - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | $ 102,424 | $ 92,668 |
Short-term investments, amortized cost | | 18,464 |
Short-term investments, unrealized gains | | 0 |
Short-term investments, unrealized losses | | (9) |
Short-term investments, total estimated fair value | | 18,455 |
Fair Value, Inputs, Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 57,969 | 52,267 |
Short-term investments, estimated fair value | | 0 |
Fair Value, Inputs, Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 44,455 | 40,401 |
Short-term investments, estimated fair value | | 18,455 |
Commercial paper | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 16,230 | 13,035 |
Commercial paper | Fair Value, Inputs, Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 0 | 0 |
Commercial paper | Fair Value, Inputs, Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 16,230 | 13,035 |
Corporate bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | | 1,205 |
Corporate bonds | Fair Value, Inputs, Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | | 0 |
Corporate bonds | Fair Value, Inputs, Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | | 1,205 |
Money market funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 57,969 | 52,267 |
Money market funds | Fair Value, Inputs, Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 57,969 | 52,267 |
Money market funds | Fair Value, Inputs, Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 0 | 0 |
Municipal bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 2,251 | 13,060 |
Municipal bonds | Fair Value, Inputs, Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 0 | 0 |
Municipal bonds | Fair Value, Inputs, Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 2,251 | 13,060 |
U.S. government and agency securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 25,974 | 13,101 |
U.S. government and agency securities | Fair Value, Inputs, Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 0 | 0 |
U.S. government and agency securities | Fair Value, Inputs, Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 25,974 | 13,101 |
Municipal bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, amortized cost | 9,032 | 11,888 |
Short-term investments, unrealized gains | 0 | 0 |
Short-term investments, unrealized losses | (5) | (3) |
Short-term investments, total estimated fair value | 9,027 | 11,885 |
Municipal bonds | Fair Value, Inputs, Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, estimated fair value | 0 | 0 |
Municipal bonds | Fair Value, Inputs, Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, estimated fair value | $ 9,027 | 11,885 |
Commercial paper | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, amortized cost | | 1,994 |
Short-term investments, unrealized gains | | 0 |
Short-term investments, unrealized losses | | 0 |
Short-term investments, total estimated fair value | | 1,994 |
Commercial paper | Fair Value, Inputs, Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, estimated fair value | | 0 |
Commercial paper | Fair Value, Inputs, Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, estimated fair value | | 1,994 |
Corporate bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, amortized cost | | 3,074 |
Short-term investments, unrealized gains | | 0 |
Short-term investments, unrealized losses | | (3) |
Short-term investments, total estimated fair value | | 3,071 |
Corporate bonds | Fair Value, Inputs, Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, estimated fair value | | 0 |
Corporate bonds | Fair Value, Inputs, Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, estimated fair value | | 3,071 |
U.S. government and agency securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, amortized cost | | 1,508 |
Short-term investments, unrealized gains | | 0 |
Short-term investments, unrealized losses | | (3) |
Short-term investments, total estimated fair value | | 1,505 |
U.S. government and agency securities | Fair Value, Inputs, Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, estimated fair value | | 0 |
U.S. government and agency securities | Fair Value, Inputs, Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, estimated fair value | | $ 1,505 |