Cash Equivalents and Investments - Schedule of Securities Classified as Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Dec. 31, 2015 |
Cash Cash Equivalents And Short Term Investments [Line Items] | | | | | |
Cash and cash equivalents, Amortized Cost | $ 59,454 | $ 74,598 | [1] | $ 160,360 | $ 107,004 |
Cash and cash equivalents, Gross Unrealized Gains | 0 | 0 | | | |
Cash and cash equivalents, Gross Unrealized Losses | 0 | 0 | | | |
Cash and cash equivalents, Fair Value | 59,454 | 74,598 | | | |
Short-term investments, Amortized Cost | 69,844 | 126,296 | | | |
Short-term investments, Gross Unrealized Gains | 3 | 7 | | | |
Short-term investments, Gross Unrealized Losses | (13) | (78) | | | |
Short-term investments, Fair Value | 69,834 | 126,225 | | | |
Cash equivalents and short-term investments, Amortized Cost | 129,298 | 200,894 | | | |
Cash equivalents and short-term investments, Gross Unrealized Gains | 3 | 7 | | | |
Cash equivalents and short-term investments, Gross Unrealized Losses | (13) | (78) | | | |
Cash equivalents and short-term investments, Fair Value | 129,288 | 200,823 | | | |
Cash [Member] | | | | | |
Cash Cash Equivalents And Short Term Investments [Line Items] | | | | | |
Cash and cash equivalents, Amortized Cost | 7,254 | 3,638 | | | |
Cash and cash equivalents, Gross Unrealized Gains | 0 | 0 | | | |
Cash and cash equivalents, Gross Unrealized Losses | 0 | 0 | | | |
Cash and cash equivalents, Fair Value | 7,254 | 3,638 | | | |
Money Market Funds [Member] | | | | | |
Cash Cash Equivalents And Short Term Investments [Line Items] | | | | | |
Cash and cash equivalents, Amortized Cost | 44,155 | 68,561 | | | |
Cash and cash equivalents, Gross Unrealized Gains | 0 | 0 | | | |
Cash and cash equivalents, Gross Unrealized Losses | 0 | 0 | | | |
Cash and cash equivalents, Fair Value | 44,155 | 68,561 | | | |
U.S. Treasury Securities [Member] | | | | | |
Cash Cash Equivalents And Short Term Investments [Line Items] | | | | | |
Cash and cash equivalents, Amortized Cost | 1,799 | | | | |
Cash and cash equivalents, Gross Unrealized Gains | 0 | | | | |
Cash and cash equivalents, Gross Unrealized Losses | 0 | | | | |
Cash and cash equivalents, Fair Value | 1,799 | | | | |
U.S. Government Sponsored Agency Bonds [Member] | | | | | |
Cash Cash Equivalents And Short Term Investments [Line Items] | | | | | |
Cash and cash equivalents, Amortized Cost | 1,800 | | | | |
Cash and cash equivalents, Gross Unrealized Gains | 0 | | | | |
Cash and cash equivalents, Gross Unrealized Losses | 0 | | | | |
Cash and cash equivalents, Fair Value | 1,800 | | | | |
Corporate Bonds [Member] | | | | | |
Cash Cash Equivalents And Short Term Investments [Line Items] | | | | | |
Cash and cash equivalents, Amortized Cost | 2,246 | | | | |
Cash and cash equivalents, Gross Unrealized Gains | 0 | | | | |
Cash and cash equivalents, Gross Unrealized Losses | 0 | | | | |
Cash and cash equivalents, Fair Value | 2,246 | | | | |
Commercial Paper (Cash Equivalents) [Member] | | | | | |
Cash Cash Equivalents And Short Term Investments [Line Items] | | | | | |
Cash and cash equivalents, Amortized Cost | 2,200 | 2,399 | | | |
Cash and cash equivalents, Gross Unrealized Gains | 0 | 0 | | | |
Cash and cash equivalents, Gross Unrealized Losses | 0 | 0 | | | |
Cash and cash equivalents, Fair Value | 2,200 | 2,399 | | | |
U.S. Treasury Securities [Member] | | | | | |
Cash Cash Equivalents And Short Term Investments [Line Items] | | | | | |
Short-term investments, Amortized Cost | 2,988 | | | | |
Short-term investments, Fair Value | 2,988 | | | | |
Corporate Bonds [Member] | | | | | |
Cash Cash Equivalents And Short Term Investments [Line Items] | | | | | |
Short-term investments, Amortized Cost | 38,247 | 58,464 | | | |
Short-term investments, Gross Unrealized Gains | 1 | 2 | | | |
Short-term investments, Gross Unrealized Losses | (9) | (56) | | | |
Short-term investments, Fair Value | 38,239 | 58,410 | | | |
Commercial Paper (Investments) [Member] | | | | | |
Cash Cash Equivalents And Short Term Investments [Line Items] | | | | | |
Short-term investments, Amortized Cost | 21,382 | 62,946 | | | |
Short-term investments, Gross Unrealized Gains | 1 | 5 | | | |
Short-term investments, Gross Unrealized Losses | (3) | (20) | | | |
Short-term investments, Fair Value | 21,380 | 62,931 | | | |
Asset-Backed Securities [Member] | | | | | |
Cash Cash Equivalents And Short Term Investments [Line Items] | | | | | |
Short-term investments, Amortized Cost | 7,227 | 4,886 | | | |
Short-term investments, Gross Unrealized Gains | 1 | | | | |
Short-term investments, Gross Unrealized Losses | (1) | (2) | | | |
Short-term investments, Fair Value | $ 7,227 | $ 4,884 | | | |
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[1] | Derived from the audited consolidated financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2016. |