Summary of Significant Accounting Policies - Internal-Use Software Development Costs (Details) - Capitalized internal-use software costs - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
| Internal-Use Software Development Costs | |||
| Capitalized development costs | $ 6.5 | $ 4.0 | $ 2.0 |
| Amortization expense | 2.4 | 1.3 | $ 0.8 |
| Net book value | $ 9.8 | $ 5.6 | |