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Share-Based Compensation - Performance Restricted Stock Units - Narrative (Details) - PRSUs shares in Thousands, $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 31, 2020 shares | Dec. 31, 2019 shares | Sep. 30, 2021 USD ($) shares | Sep. 30, 2020 USD ($) shares | Sep. 30, 2021 USD ($) metric shares | Sep. 30, 2020 USD ($) shares | Dec. 31, 2020 metric shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Number of performance award metrics | metric |  |  |  |  |  |  | 4
Number of units increased (decreased) during the period (in shares) | shares | (125) | 1 |  |  | (2) |  |
Recognized share-based compensation expense (benefit) | $ |  |  | $ 0.3 | $ 0.2 | $ 0.7 | $ 0.8 |
Unrecognized compensation cost | $ |  |  | $ 1.6 |  | $ 1.6 |  |
Nonvested shares outstanding (in shares) | shares | 333 | 260 | 305 | 329 | 305 | 329 | 333
Weighted-average period over which unrecognized compensation cost is expected to be recognized (years) |  |  |  |  | 2 years 2 months 12 days |  |
Executive and Senior Officers |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Performance period (years) |  |  |  |  | 1 year |  |
Requisite service period |  |  |  |  | 5 years |  |
Executive and Senior Officers | Anniversary 1 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting percentage (ratably on each of the first four anniversary date) |  |  |  |  | 25.00% |  |
Executive and Senior Officers | Anniversary 2 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting percentage (ratably on each of the first four anniversary date) |  |  |  |  | 25.00% |  |
Executive and Senior Officers | Anniversary 3 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting percentage (ratably on each of the first four anniversary date) |  |  |  |  | 25.00% |  |
Executive and Senior Officers | Anniversary 4 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting percentage (ratably on each of the first four anniversary date) |  |  |  |  | 25.00% |  |
Executive Officers |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Performance period (years) |  |  |  |  | 1 year |  |
Number of performance award metrics | metric |  |  |  |  | 3 |  |
Average share price trading period |  |  |  |  | 20 days |  | 20 days
Last days trading period |  |  |  |  | 20 days |  | 20 days
Dividend yield |  |  |  |  | 0.00% |  |

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