Investments - Additional Information (Details) - USD ($) |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2018 |
Jun. 30, 2017 |
Jun. 30, 2018 |
Jun. 30, 2017 |
|
| Cash Cash Equivalents And Available For Sale Securities [Abstract] | ||||
| Realized gains and losses on available-for-sale securities | $ 0 | $ 0 | $ 0 | $ 0 |
| Contractual maturity date of investments | Less than one year | |||