Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	43,105
Assumed forfeitures (in shares)	(647)
Total target performance shares outstanding (in shares)	42,458	42,458	42,473	42,473	42,473	40,616
Accrued Expense	$ 29.6
Assumed forfeitures	(0.5)
Total Accrued Expense	$ 29.1	$ 15.8	$ 43.7	$ 36.4	$ 30.8	$ 31.7
Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,450
Accrued Expense	$ 15.4
Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	15,600
Accrued Expense	$ 10.6
Performance Cycle 2020 to 2022
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	14,055
Accrued Expense	$ 3.6