Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - Restricted Stock - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Restricted Shares
Beginning of period (in shares)	43,105	43,395	41,510
Issued (in shares)	13,475	14,055	15,600
Vested (in shares)	(17,936)	(14,345)	(13,715)
Forfeited (in shares)	(794)	0	0
Expense recognized (in shares)	0	0	0
End of period (in shares)	37,850	43,105	43,395
Unamortized Issue Date Fair Value
Beginning of period	$ 15.2	$ 16.7	$ 12.5
Issued	16.1	15.1	14.5
Vested	0.0	0.0	0.0
Forfeited	(0.8)	0.0	0.0
Expense recognized	(14.6)	(16.6)	(10.3)
End of period	$ 15.9	$ 15.2	$ 16.7