Fair Value Measurements - Available For Sale (Details) - USD ($) $ in Thousands |
Dec. 31, 2016 |
Dec. 31, 2015 |
|---|---|---|
| Fair value of available for sale marketable securities | ||
| Fair Value | $ 129,846 | $ 90,281 |
| U.S. Treasury securities | ||
| Fair value of available for sale marketable securities | ||
| Amortized Cost | 129,885 | 90,326 |
| Gross Unrealized Losses | (39) | (45) |
| Fair Value | $ 129,846 | $ 90,281 |