Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	May. 11, 2015
Class of Stock [Line Items]
Shares issued under dividend reinvestment plan (in shares)	16,524	18,705	20,026
Preferred shares, shares outstanding	399,896	399,896	399,896
Series 1 Cumulative Convertible Preferred interest rate	5.417%	5.417%	5.417%
Preferred shares, liquidation preference per share	$ 25	$ 25	$ 25
Preferred shares, par value	0.01	$ 0.01	$ 0.01
Conversion price per share	$ 104.69
Net cash proceeds of common stock	$ 110,855	$ 216,155	$ 186,548
Common shares of beneficial interest, par	$ 0.01	$ 0.01
At The Market Equity Program
Class of Stock [Line Items]
Aggregate offering price of common shares				$ 300,000
Shares issued under equity program	813,414	1,768,583
Weighted average price per common share	$ 135.01	$ 122.09
Net cash proceeds of common stock	$ 108,500	$ 213,600
Remaining capacity to issue	190,200
At The Market Equity Program | Commissions
Class of Stock [Line Items]
Commissions related to sales of common shares	1,100	2,200
At The Market Equity Program | Other Offering Costs
Class of Stock [Line Items]
Commissions related to sales of common shares	$ 200	$ 200