Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) multiplier shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares
Performance Shares
Share-based compensation arrangement by share-based payment award
Multiplier for determining payout, low end of range | multiplier	0
Multiplier for determining payout, high end of range | multiplier	2
Performance cycle period	3 years
Unamortized grant date fair value | $	$ 28.8
Performance Share Roll forward
Beginning of period (in shares)	42,473	40,616	50,515
Shares paid or expired (in shares)	(14,070)	(13,715)	(23,186)
New grants (in shares)	14,055	15,600	14,105
Forfeitures (in shares)	0	(28)	(818)
Expense recognized (in shares)	0	0	0
End of period (in shares)	42,458	42,473	40,616
Unamortized Issue Date Fair Value
Beginning of period | $	$ 43.7	$ 31.7	$ 45.8
Shares paid or expired | $	(27.7)	(18.1)	(28.4)
New grants | $	0.0	0.0	0.0
Forfeitures | $	(0.4)	(0.1)	0.1
Expense recognized | $	40.7	30.2	14.2
End of period | $	$ 56.3	$ 43.7	$ 31.7
Performance Shares | Performance Cycle 2020 to 2022
Unamortized Issue Date Fair Value
Targeted performance goal (as a percent)	7.00%
Payout for maximum growth target percentage	200.00%
Performance Shares | W T M Incentive and Phantom Plan | Range Of Performance Cycle From 2017 to 2019
Unamortized Issue Date Fair Value
Payout for minimum growth target percentage	174.00%
Payout for maximum growth target percentage	180.00%
Performance Shares | W T M Incentive and Phantom Plan | Range Of Performance Cycle From 2016-2018
Unamortized Issue Date Fair Value
Payout for minimum growth target percentage		139.00%
Payout for maximum growth target percentage		166.00%
Performance Shares | W T M Incentive and Phantom Plan | Range Of Performance Cycle From 2015 to 2017
Unamortized Issue Date Fair Value
Payout for minimum growth target percentage			145.00%
Payout for maximum growth target percentage			147.00%
Restricted Share Cliff Vesting in January 2022 | W T M Incentive and Phantom Plan
Share-based compensation arrangement by share-based payment award
Performance shares (in shares)	14,055	15,600
Restricted Share Cliff Vesting in January 2021 | W T M Incentive and Phantom Plan
Share-based compensation arrangement by share-based payment award
Performance shares (in shares)		15,600
Restricted Share Cliff Vesting in January 2020 | W T M Incentive and Phantom Plan
Share-based compensation arrangement by share-based payment award
Performance shares (in shares)			13,450
Restricted Share Cliff Vesting in January 2019 | W T M Incentive and Phantom Plan
Share-based compensation arrangement by share-based payment award
Performance shares (in shares)			290
Restricted Share Cliff Vesting in January 2018 | W T M Incentive and Phantom Plan
Share-based compensation arrangement by share-based payment award
Performance shares (in shares)			365