Incentive Award Plan and Employee Benefit Plan - Summary of Performance Share Award Activity under Plan (Details) - USD ($)
$ / shares in Units, $ in Millions |
12 Months Ended |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
| Weighted Average Fair Value |
|
|
|
| Shares released at target |
466,667
|
155,782
|
134,932
|
| Cash paid for dividend rights |
$ 0.5
|
$ 0.2
|
$ 1.1
|
| Performance Shares |
|
|
|
| Number of Target Shares |
|
|
|
| Outstanding non-vested shares, beginning of year (in shares) |
494,300
|
472,725
|
610,797
|
| Shares granted (in shares) |
858,226
|
400,800
|
279,199
|
| Shares earned above performance target (in shares) |
0
|
(42,640)
|
387,027
|
| Shares vested (in shares) |
(466,667)
|
(215,438)
|
(804,298)
|
| Shares forfeited (in shares) |
(42,373)
|
(121,147)
|
0
|
| Outstanding non-vested shares, end of year (in shares) |
843,486
|
494,300
|
472,725
|
| Weighted Average Fair Value |
|
|
|
| Outstanding non-vested shares, beginning of year (in dollars per share) |
$ 15.56
|
$ 14.28
|
$ 13.49
|
| Shares granted (in dollars per share) |
11.88
|
16.06
|
14.78
|
| Shares earned above performance target (in dollars per share) |
0.00
|
13.56
|
13.45
|
| Shares vested (in dollars per share) |
14.48
|
14.36
|
13.46
|
| Shares forfeited (in dollars per share) |
14.78
|
15.05
|
0.00
|
| Outstanding non-vested shares, end of year (in dollars per share) |
$ 12.45
|
$ 15.56
|
$ 14.28
|