Shareholders' Equity (Summary of Dividends) (Details) - $ / shares		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Dividends
Dividends declared to common shareholders (in dollars per share)		$ 1.06	$ 1.05	$ 2.12	$ 2.100
Dividends paid per common share				2.120	2.100
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				0.677	0.677
Dividends paid per preferred share				0.677	0.677
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]			0.625	0.625
Dividends paid per preferred share	[1]			$ 0.625	$ 0.625

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