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

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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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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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Number of shares issued under share-based payment arrangement.

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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		9 Months Ended
	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($) shares	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 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) \| shares	0	0	13,475	14,055
Additional compensation cost that would be recognized if all outstanding performance shares vested \| $	$ 17.2		$ 17.2
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 \| $	$ 19.4	$ 19.4	$ 19.4	$ 19.4	$ 23.2	$ 15.2	$ 23.0	$ 16.7
Issued (in shares) \| shares	0	0	13,475	14,055