Shareholders' Equity (Summary of Dividends) (Details) - $ / shares		3 Months Ended		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Dividends
Dividends declared per common share		$ 1.00	$ 0.980
Dividends paid per common share		$ 1.00	$ 0.980
Preferred shares, percentage		5.417%	5.417%
5.417% Series 1 Cumulative Convertible Preferred
Dividends
Preferred shares, percentage		5.417%	5.417%	5.417%
Dividends declared per preferred share		$ 0.339	$ 0.339
Dividends paid per preferred share		$ 0.339	$ 0.339
5.0% Series C Cumulative Redeemable Preferred
Dividends
Preferred shares, percentage		5.00%	5.00%	5.00%
Dividends declared per preferred share	[1]	$ 0.313	$ 0
Dividends paid per preferred share	[1]	$ 0.368	$ 0

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