Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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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.

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Represents the performance cycle on conclusion of which shares become payable.

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Share-based Compensation Arrangement by Share-based Payment Award, Multiplier for Determining Payout

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Employee Share-Based Incentive Compensation Plans - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) shares	Mar. 31, 2020 USD ($) shares	Dec. 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
Performance shares (in shares)	41,405
New grants (in shares)	13,000	14,055
Additional compensation cost that would be recognized if all outstanding performance shares vested \| $	$ 38.1
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 Share Cliff Vesting in January 2023 \| W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Performance shares (in shares)	13,000
Restricted Share Cliff Vesting in January 2022 \| W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
New grants (in shares)		14,055
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 \| $	$ 26.2	$ 26.6	$ 15.2	$ 16.7