Segment and Geographic Information - Geographical Information (Details) - USD ($) $ in Thousands |
3 Months Ended | 12 Months Ended | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Dec. 31, 2017 |
Sep. 30, 2017 |
Jun. 30, 2017 |
Mar. 31, 2017 |
Dec. 31, 2016 |
Sep. 30, 2016 |
Jun. 30, 2016 |
Mar. 31, 2016 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Geographical Information | |||||||||||
| Revenue | $ 86,678 | $ 70,250 | $ 64,995 | $ 64,829 | $ 57,350 | $ 51,960 | $ 49,110 | $ 47,444 | $ 286,752 | $ 205,864 | $ 150,194 |
| Short Course Segment | |||||||||||
| Geographical Information | |||||||||||
| Revenue | 16,320 | ||||||||||
| Short Course Segment | Non-US | |||||||||||
| Geographical Information | |||||||||||
| Revenue | 10,000 | ||||||||||
| Long-lived assets | $ 700 | $ 700 | |||||||||