Incentive Award Plan - Narrative (Details) - USD ($)
shares in Millions, $ in Millions |
3 Months Ended |
|
Mar. 31, 2018 |
Mar. 31, 2017 |
Dec. 31, 2017 |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Unamortized stock-based compensation expense |
$ 25.4
|
|
|
| General and administrative expense |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Stock-based compensation expense |
4.4
|
$ 2.2
|
|
| Non-vested Shares of Restricted Stock |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Deferred compensation expense |
$ 7.0
|
|
|
| Outstanding unvested shares |
2.1
|
|
|
| Unamortized stock-based compensation expense |
$ 14.4
|
|
|
| Non-vested Shares of Restricted Stock | Executives Officers and Employees |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Grants in period (in shares) |
0.9
|
|
|
| Performance Shares |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Approved initial target grant (in shares) |
0.5
|
|
|
| Minimum required service period |
3 years
|
|
|
| Dividend rights accrued |
$ 1.1
|
|
$ 0.8
|
| Potential grants in period based on total shareholder return (in shares) |
1.8
|
|
|
| Unamortized stock-based compensation expense |
$ 11.0
|
|
|
| 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%
|
|
|