Incentive Award Plan and Employee Benefit Plan - Summary of Market-Based Award Activity under Plan (Details) - USD ($)
$ / shares in Units, $ in Millions |
12 Months Ended |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
| Weighted Average Fair Value |
|
|
|
| Shares released at target |
27,267
|
93,333
|
31,156
|
| Cash paid for dividend rights |
$ 0.1
|
$ 0.5
|
$ 0.2
|
| Market-Based Awards |
|
|
|
| Number of Target Shares |
|
|
|
| Outstanding non-vested shares, beginning of year (in shares) |
168,694
|
98,859
|
94,542
|
| Shares granted (in shares) |
100,899
|
171,642
|
80,158
|
| Shares earned above performance target (in shares) |
0
|
0
|
(8,528)
|
| Shares vested (in shares) |
(27,267)
|
(93,333)
|
(43,085)
|
| Shares forfeited (in shares) |
(2,168)
|
(8,474)
|
(24,228)
|
| Incremental shares (in shares) |
26,643
|
0
|
0
|
| Outstanding non-vested shares, end of year (in shares) |
266,801
|
168,694
|
98,859
|
| Weighted Average Fair Value |
|
|
|
| Outstanding non-vested shares, beginning of year (in dollars per share) |
$ 62.25
|
$ 77.79
|
$ 71.38
|
| Shares granted (in dollars per share) |
51.98
|
59.38
|
80.30
|
| Shares earned above performance target (in dollars per share) |
0.00
|
0.00
|
67.80
|
| Shares vested (in dollars per share) |
70.24
|
72.38
|
71.80
|
| Shares forfeited (in dollars per share) |
80.32
|
73.90
|
75.25
|
| Outstanding non-vested shares, end of year (in dollars per share) |
$ 51.19
|
$ 62.25
|
$ 77.79
|