Compensation Arrangements - 401(k) Defined Contribution Plan (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Defined Benefit Plan Disclosure [Line Items] | |||
| Maximum 401 (k) plan contribution rates as percentage of employees earnings | 75.00% | ||
| Employer matching contribution, maximum (percentage) | 100.00% | ||
| Initial employer 401(k) matching contribution to employee (percentage) | 4.00% | ||
| Employer 401 (k) matching contribution to employee, 50% Maximum | 50.00% | ||
| Compensation expense related to 401(k) savings plan | $ 6.9 | $ 5.7 | $ 5.4 |
| Maximum | |||
| Defined Benefit Plan Disclosure [Line Items] | |||
| Initial employer 401(k) matching contribution to employee (percentage) | 3.00% | ||
| Minimum | |||
| Defined Benefit Plan Disclosure [Line Items] | |||
| Initial employer 401(k) matching contribution to employee (percentage) | 2.00% | ||