Cash, Cash Equivalents and Investments in Marketable Securities - Summary of Cash Equivalents and Investments Carried at Fair Value (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Dec. 31, 2016 |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Cash and restricted cash | $ 11,997 | $ 11,686 | $ 13,327 | $ 15,038 |
Available for sale securities | 17,514 | 650 | | |
Commercial Paper [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 3,237 | | | |
Corporate Notes and Bonds [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 6,491 | | | |
U.S Treasuries [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 7,786 | | | |
Estimate of Fair Value Measurement [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Cash and restricted cash | 3,238 | 4,587 | | |
Available for sale securities | 26,273 | 7,064 | | |
Estimate of Fair Value Measurement [Member] | Money Market Funds Included In Cash Equivalents [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 8,759 | 6,414 | | |
Estimate of Fair Value Measurement [Member] | US Government Agencies Debt Securities Included in Cash Equivalents [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | | 650 | | |
Estimate of Fair Value Measurement [Member] | Commercial Paper [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 3,237 | | | |
Estimate of Fair Value Measurement [Member] | Corporate Notes and Bonds [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 6,491 | | | |
Estimate of Fair Value Measurement [Member] | U.S Treasuries [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 7,786 | | | |
Estimate of Fair Value Measurement [Member] | Level I [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 16,545 | 6,414 | | |
Estimate of Fair Value Measurement [Member] | Level I [Member] | Money Market Funds Included In Cash Equivalents [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 8,759 | 6,414 | | |
Estimate of Fair Value Measurement [Member] | Level I [Member] | US Government Agencies Debt Securities Included in Cash Equivalents [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | | 0 | | |
Estimate of Fair Value Measurement [Member] | Level I [Member] | Commercial Paper [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 0 | | | |
Estimate of Fair Value Measurement [Member] | Level I [Member] | Corporate Notes and Bonds [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 0 | | | |
Estimate of Fair Value Measurement [Member] | Level I [Member] | U.S Treasuries [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 7,786 | | | |
Estimate of Fair Value Measurement [Member] | Level II [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 9,728 | 650 | | |
Estimate of Fair Value Measurement [Member] | Level II [Member] | Money Market Funds Included In Cash Equivalents [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 0 | 0 | | |
Estimate of Fair Value Measurement [Member] | Level II [Member] | US Government Agencies Debt Securities Included in Cash Equivalents [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | | 650 | | |
Estimate of Fair Value Measurement [Member] | Level II [Member] | Commercial Paper [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 3,237 | | | |
Estimate of Fair Value Measurement [Member] | Level II [Member] | Corporate Notes and Bonds [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 6,491 | | | |
Estimate of Fair Value Measurement [Member] | Level II [Member] | U.S Treasuries [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 0 | | | |
Estimate of Fair Value Measurement [Member] | Level III [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 0 | 0 | | |
Estimate of Fair Value Measurement [Member] | Level III [Member] | Money Market Funds Included In Cash Equivalents [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 0 | 0 | | |
Estimate of Fair Value Measurement [Member] | Level III [Member] | US Government Agencies Debt Securities Included in Cash Equivalents [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | | $ 0 | | |
Estimate of Fair Value Measurement [Member] | Level III [Member] | Commercial Paper [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 0 | | | |
Estimate of Fair Value Measurement [Member] | Level III [Member] | Corporate Notes and Bonds [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | 0 | | | |
Estimate of Fair Value Measurement [Member] | Level III [Member] | U.S Treasuries [Member] | | | | |
Schedule Of Cash Equivalents And Available For Sale Investments [Line Items] | | | | |
Available for sale securities | $ 0 | | | |