Stock-Based Compensation - Assumptions Used in Granting Stock Options (Details) - Stock option awards - $ / shares |
3 Months Ended | 12 Months Ended | |
|---|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Weighted average grant date fair value (in dollars per share) | $ 5.93 | $ 8.58 | $ 7.15 |
| Expected term (in years) | 6 years | 6 years | 6 years |
| Expected stock volatility | 40.00% | 42.00% | 38.00% |
| Dividend yield | 0.00% | 0.00% | 0.00% |
| Risk-free interest rate | 1.50% | 2.70% | 2.00% |