Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted Stock
Restricted Share Roll forward
Beginning of period (in shares)	43,395	41,510	53,755
Issued (in shares)	14,055	15,600	14,105
Vested (in shares)	(14,345)	(13,715)	(25,381)
Assumed forfeitures and cancellations (in shares)	0	0	(969)
Expense recognized (in shares)	0	0	0
End of period (in shares)	43,105	43,395	41,510
Unamortized Issue Date Fair Value
Beginning of period	$ 16.7	$ 12.5	$ 14.3
Issued	15.1	14.5	11.4
Vested	0.0	0.0	0.0
Forfeited	0.0	0.0	(0.2)
Expense recognized	(16.6)	(10.3)	(13.0)
End of period	$ 15.2	$ 16.7	$ 12.5
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2023
Restricted Share Roll forward
Issued (in shares)	14,055
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2022
Restricted Share Roll forward
Issued (in shares)	14,055	15,600
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2021
Restricted Share Roll forward
Issued (in shares)		15,600
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2019
Restricted Share Roll forward
Issued (in shares)			290
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2018
Restricted Share Roll forward
Issued (in shares)			365
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2020
Restricted Share Roll forward
Issued (in shares)			13,450