Shareholders' Equity (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 12, 2014
Class of Stock [Line Items]
Shares issued under dividend reinvestment plan (in shares)	18,705	20,026	22,814
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	$ 216,155,000	$ 186,548,000	$ 112,270,000
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				300,000,000
Shares issued under equity program	1,768,583	1,734,974
Weighted average price per common share	$ 122.09	$ 108.01
Net cash proceeds of common stock	213,600,000	185,200,000
Remaining capacity to issue	84,100,000
At The Market Equity Program | Commissions
Class of Stock [Line Items]
Commissions related to sales of common shares	2,200,000	2,000,000
At The Market Equity Program | Other Offering Costs
Class of Stock [Line Items]
Commissions related to sales of common shares	$ 200,000	$ 200,000