Intangible Assets - Narrative (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2018 |
Sep. 30, 2017 |
Sep. 30, 2018 |
Sep. 30, 2017 |
|
| Finite-Lived Intangible Assets [Line Items] | ||||
| Increase in amortization expense | $ 0.6 | $ 2.3 | ||
| Course creation costs | ||||
| Finite-Lived Intangible Assets [Line Items] | ||||
| Amortization of acquired intangible assets | 0.5 | $ 0.4 | 1.4 | $ 1.1 |
| Other Intangible Assets | ||||
| Finite-Lived Intangible Assets [Line Items] | ||||
| Amortization of acquired intangible assets | $ 1.1 | $ 2.0 | $ 7.7 | $ 6.0 |