Shareholders' Equity (Summary of Dividends) (Details) - $ / shares		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Dividends
Dividends declared per common share				$ 2.00	$ 1.96
Dividends paid per common share				$ 2.00	$ 1.96
Preferred shares, percentage		5.417%	5.417%	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 shares				$ 0.677	$ 0.677
Dividends paid per preferred share				$ 0.677	$ 0.677
5.0% Series C Cumulative Redeemable Preferred
Dividends
Preferred shares, percentage				5.00%	5.00%	5.00%
Dividends declared per preferred shares	[1]			$ 0.625	$ 0
Dividends paid per preferred share	[1]			$ 0.681	$ 0

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