Investments - Summary of Fair Value of Available for Sale Investments (Detail) - USD ($) $ in Thousands |
Dec. 31, 2017 |
Dec. 31, 2016 |
---|---|---|
Schedule of Available-for-sale Securities [Line Items] | ||
Investments, Amortized Cost | $ 38,922 | |
Investments, Gross Unrealized Loss | (33) | |
Investments, Fair Value | 38,889 | $ 0 |
Corporate Notes [Member] | ||
Schedule of Available-for-sale Securities [Line Items] | ||
Investments, Amortized Cost | 25,733 | |
Investments, Gross Unrealized Loss | (23) | |
Investments, Fair Value | 25,710 | |
Commercial Paper [Member] | ||
Schedule of Available-for-sale Securities [Line Items] | ||
Investments, Amortized Cost | 13,189 | |
Investments, Gross Unrealized Loss | (10) | |
Investments, Fair Value | $ 13,179 |