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Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Target Performance Shares Outstanding
Beginning of period (in shares)	42,458	42,473
Shares paid (in shares)	(14,117)	(14,070)
New grants (in shares)	13,000	14,055
Forfeitures and cancellations (in shares)	(557)	0
Expense recognized (in shares)	0	0
End of period (in shares)	40,784	42,458
Accrued Expense
Beginning of period	$ 56.3	$ 43.7
Shares paid	(34.6)	(27.7)
New grants	0.0	0.0
Forfeitures and cancellations	0.4	0.4
Expense recognized	16.1	(6.6)
End of period	$ 38.2	$ 9.8
Performance Cycle 2018 to 2020
Accrued Expense
Percentage of target payout - minimum actual	200.00%
Performance Cycle 2017 to 2019
Accrued Expense
Percentage of target payout - minimum actual		174.00%
Percentage of target payout - maximum actual		180.00%

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- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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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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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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