Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details) - Fair Value Recurring Basis $ in Thousands |
Dec. 31, 2017
USD ($)
|
|---|---|
| Assets: | |
| Marketable securities | $ 17,631 |
| Restricted cash and investments | 13,153 |
| Level 1 | |
| Assets: | |
| Marketable securities | 7,906 |
| Restricted cash and investments | 9,055 |
| Level 2 | |
| Assets: | |
| Marketable securities | 9,725 |
| Restricted cash and investments | $ 4,098 |