Incentive Award Plan - Narrative (Details) - USD ($)
shares in Thousands, $ in Millions |
3 Months Ended |
6 Months Ended |
|
Jun. 30, 2019 |
Jun. 30, 2018 |
Jun. 30, 2019 |
Jun. 30, 2018 |
Dec. 31, 2018 |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
|
| Unamortized stock-based compensation expense |
$ 19.5
|
|
$ 19.5
|
|
$ 15.5
|
| General and administrative expense |
|
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
|
| Stock-based compensation expense |
$ 3.9
|
$ 4.7
|
7.5
|
$ 9.1
|
|
| Non-vested Shares of Restricted Stock |
|
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
|
| Deferred compensation expense |
|
|
$ 6.6
|
|
|
| Outstanding unvested shares |
399
|
|
399
|
|
|
| Unamortized stock-based compensation expense |
$ 10.3
|
|
$ 10.3
|
|
8.1
|
| Performance Shares |
|
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
|
| Outstanding unvested shares |
500
|
|
500
|
|
|
| Number of shares available for grant |
97
|
|
97
|
|
|
| Volatility rate |
|
|
25.40%
|
|
|
| Minimum required service period |
|
|
3 years
|
|
|
| Dividend rights accrued |
$ 2.3
|
|
$ 2.3
|
|
1.7
|
| Unamortized stock-based compensation expense |
$ 9.2
|
|
$ 9.2
|
|
$ 7.4
|
| Performance Shares | Minimum |
|
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
|
| Multiplier for shares granted |
|
|
0.00%
|
|
|
| Performance Shares | Maximum |
|
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
|
| Multiplier for shares granted |
|
|
250.00%
|
|
|
| Performance Shares | Group of Industry Peers |
|
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
|
| Volatility rate, minimum |
|
|
15.30%
|
|
|
| Volatility rate, maximum |
|
|
30.80%
|
|
|
| Executives Officers and Employees | Non-vested Shares of Restricted Stock |
|
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
|
| Grants in period (in shares) |
|
|
173
|
|
|