Incentive Award Plan and Employee Benefit Plan - Summary of Restricted Share Grant Activity under Plan (Details) - Unvested shares of restricted stock, less shares assumed repurchased at market - $ / shares |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Number of Target Shares | |||
| Outstanding non-vested shares, beginning of year (in shares) | 286,917 | 206,899 | 154,177 |
| Shares granted (in shares) | 207,253 | 220,712 | 189,758 |
| Shares vested (in shares) | (137,292) | (131,534) | (112,516) |
| Shares forfeited (in shares) | (10,697) | (9,160) | (24,520) |
| Outstanding non-vested shares, end of year (in shares) | 346,181 | 286,917 | 206,899 |
| Weighted average price | |||
| Outstanding non-vested shares, beginning of year (in dollars per share) | $ 53.00 | $ 62.73 | $ 56.44 |
| Shares granted (in dollars per share) | 39.43 | 46.13 | 62.92 |
| Shares vested (in dollars per share) | 52.11 | 56.49 | 55.51 |
| Shares forfeited (in dollars per share) | 45.02 | 58.30 | 57.79 |
| Outstanding non-vested shares, end of year (in dollars per share) | $ 45.48 | $ 53.00 | $ 62.73 |