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Employee Share-Based Incentive Compensation Plans - Narrative (Details) $ in Millions | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2021 USD ($) shares | Jun. 30, 2020 USD ($) shares | Jun. 30, 2021 USD ($) shares | Jun. 30, 2020 USD ($) shares | Mar. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Performance Share | W T M Incentive Plan |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Vesting period |  |  | 3 years |  |  |  |  |
Performance cycle period |  |  | 3 years |  |  |  |  |
New grants (in shares) | 475 | 0 | 13,475 | 14,055 |  |  |  |
Additional compensation cost that would be recognized if all outstanding performance shares vested | $ | $ 33.4 |  | $ 33.4 |  |  |  |  |
Performance Share | Minimum | W T M Incentive Plan |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, multiplier |  |  | 0 |  |  |  |  |
Performance Share | Maximum | W T M Incentive Plan |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, multiplier |  |  | 2 |  |  |  |  |
Restricted Stock |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Vesting period |  |  | 34 months |  |  |  |  |
Restricted Stock | W T M Incentive Plan |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Additional compensation cost that would be recognized if all outstanding performance shares vested | $ | $ 23.2 | $ 23.0 | $ 23.2 | $ 23.0 | $ 26.2 | $ 15.2 | $ 26.6 | $ 16.7
Issued (in shares) | 475 | 0 | 13,475 | 14,055 |  |  |  |
Restricted Share Cliff Vesting in January 2022 | W T M Incentive Plan |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Issued (in shares) |  |  |  | 14,055 |  |  |  |

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- Definition Amount of cost not yet recognized for nonvested award under share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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