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
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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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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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).

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-Section 50
-Paragraph 2
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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This element represents the number of non-vested performance shares before forfeiture assumptions.

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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) - W T M Incentive Plan $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 shares
Performance Share
Share-based compensation arrangement by share-based payment award
Vesting period			3 years
Performance cycle period			3 years
Target performance shares outstanding (in shares)	43,105		43,105
Performance shares granted during period (in shares)	0	0	14,055	15,600
Additional compensation cost that would be recognized if all outstanding performance shares vested \| $	$ 19.4		$ 19.4
Performance Share \| Minimum
Share-based compensation arrangement by share-based payment award
Share-based compensation arrangement by share-based payment award, multiplier			0
Performance Share \| Maximum
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
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	14,055		14,055
Restricted Share Cliff Vesting in January 2022
Share-based compensation arrangement by share-based payment award
Performance shares granted during period (in shares)				15,600