Shareholders' Equity - Narrative (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended			0 Months Ended	9 Months Ended	0 Months Ended			9 Months Ended	0 Months Ended				3 Months Ended	6 Months Ended	9 Months Ended		0 Months Ended					0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Feb. 21, 2014 Common Shares	Sep. 30, 2014 Common Shares	Sep. 24, 2014 Series D	Jul. 25, 2014 Series D	Jan. 03, 2014 Series D	Sep. 30, 2014 Series D	Sep. 24, 2014 Series E	Jul. 25, 2014 Series E	Jan. 03, 2014 Series E	Jan. 28, 2014 Trustees Common Shares trustee	Sep. 30, 2014 RMR	Jun. 30, 2014 RMR	Sep. 30, 2014 RMR	Sep. 30, 2014 Equity compensation plan RMR Officers and employees	Mar. 25, 2014 Equity compensation plan RMR Officers and employees Common Shares	Jul. 25, 2014 Accrued dividend from May 15, 2014 to August 14, 2014 Series D	Jul. 25, 2014 Accrued dividend from May 15, 2014 to August 14, 2014 Series E	Jul. 25, 2014 Accrued dividend from February 15, 2014 to May 14, 2014 Series D	Jul. 25, 2014 Accrued dividend from February 15, 2014 to May 14, 2014 Series E	May 14, 2014 Fundamental change conversion right Common Shares	May 14, 2014 Fundamental change conversion right Series D
Class of Stock [Line Items]
Shares of beneficial interest granted (in shares)															2,000
Common shares of beneficial interest, par value (in dollars per share)	$ 0.01		$ 0.01		$ 0.01										$ 0.01
Closing price of common shares (in dollars per share)															$ 23.46
Number of former trustees to whom common shares granted															2
Common shares issued (in shares)																32,909	68,206	90,135
Common shares issued to holders of preferred shares (in shares)											10,412,499
Preferred shares converted into common shares (in shares)											10,264,503														10,411,779	10,263,003
Common shares previously issued (in shares)																				130,914
Compensation expense																			$ 3.4
Distributions declared per common share (in dollars per share)	$ 0	$ 0.25	$ 0.25	$ 0.75		$ 0.25
Distributions paid or declared on common shares						29.6
Dividend declared (in dollars per share)								$ 0.40625	$ 0.8125	$ 0.40625		$ 0.45313	$ 0.90625	$ 0.45313
Accrued dividend (in dollars per share)																					$ 0.40625	$ 0.453125	$ 0.40625	$ 0.453125
Liquidation preference (in dollars per share)											$ 25.00
Percentage of average closing market price of the entity's common stock based on which ratio for conversion of preferred shares into common shares is determined											98.00%
Number of consecutive trading days considered for calculating the average of the closing sale prices of the common shares											5 days
Number of shares per $25.00 liquidation preference considered for calculating the Fundamental Change Conversion Rate							1.0145
Distribution on conversion of preferred shares excess of the market value of the common shares issued above the carrying value of preferred shares																										$ 16.2
Common shares from conversion of preferred shares (in shares)							2,363,248				4,915,497