Shareholders' Equity (Summary of Dividends) (Details) - $ / shares		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Dividends
Dividends declared to common shareholders (in dollars per share)		$ 1.06	$ 1.05
Dividends paid per common share		1.060	1.050
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		0.339	0.339
Dividends paid per preferred share		0.339	0.339
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]	0.313	0.313
Dividends paid per preferred share	[1]	$ 0.313	$ 0.313

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