Shareholders' Equity (Summary of Dividends) (Details) - $ / shares		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Dividends
Dividends declared to common shareholders (in dollars per share)		$ 1.07	$ 1.06	$ 3.19	$ 3.160
Dividends paid per common share				3.180	3.150
5.417% Series 1 Cumulative Convertible Preferred Shares, (stated at liquidation preference $25 per share), 399,896 shares issued and outstanding
Dividends
Dividends declared per preferred shares				1.016	1.016
Dividends paid per preferred share				1.016	1.016
5.0% Series C Cumulative Redeemable Preferred Shares, (stated at liquidation preference $25,000 per share), 6,000 shares issued and outstanding
Dividends
Dividends declared per preferred shares	[1]			0.938	0.938
Dividends paid per preferred share	[1]			$ 0.938	$ 0.938

[1] Amount represents dividends per depository share, each representing 1/1000th of a share.