Summary of Significant Accounting Policies - Software Costs (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | |||
|---|---|---|---|---|---|
Jun. 30, 2018 |
Jun. 30, 2017 |
Jun. 30, 2018 |
Jun. 30, 2017 |
Dec. 31, 2017 |
|
| Accounting Policies [Abstract] | |||||
| Capitalized software | $ 7.5 | $ 7.5 | $ 7.3 | ||
| Capitalized software, estimated useful life | 5 years | ||||
| Software amortization expense | $ 0.8 | $ 0.6 | $ 1.4 | $ 1.2 | |