Shareholders' Equity - Narrative (Details)		1 Months Ended	3 Months Ended				12 Months Ended
	Mar. 25, 2014 USD ($) $ / shares shares	Jan. 31, 2014 USD ($) $ / shares	Dec. 31, 2014 $ / shares shares	Sep. 30, 2014 $ / shares	Jun. 30, 2014 $ / shares	Mar. 31, 2014 $ / shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 USD ($)	Sep. 14, 2015 USD ($)	Aug. 24, 2015 USD ($)
Class of Stock [Line Items]
Distributions on common stock declared (in dollars per share) | $ / shares		$ 0.25	$ 0	$ 0	$ 0	$ 0.25
Distributions paid or declared on common shares | $		$ 29,600,000
Distribution for the excess of the market value of common shares issued above carrying value of series D preferred shares redeemed | $							$ 0	$ 16,205,000	$ 0
Distribution percentage to shareholders consisting of an alternative minimum tax adjustment							0.00%	15.41%	0.00%
Common Shares
Class of Stock [Line Items]
Common shares issued to holders of preferred shares (in shares)							144	10,412,499
Stock repurchase program, authorized amount | $											$ 100,000,000.0
Stock repurchase program, additional authorized amount | $										$ 100,000,000.0
Number of shares repurchased (in shares)							3,410,300
Price per common share (in dollars per share) | $ / shares							$ 25.76
Common shares from conversion of preferred shares (in shares)							2,363,103
Common Shares | Fundamental change conversion right
Class of Stock [Line Items]
Preferred shares converted into common shares (in shares)	10,411,779
Series D
Class of Stock [Line Items]
Number of shares repurchased (in shares)							301	10,264,503
Preferred shares, of beneficial interest, shares outstanding (in shares)			4,915,497				4,915,196	4,915,497
Preferred shares dividend (in dollars per share) | $ / shares							$ 1.625
Preferred shares, dividend yield							6.50%	6.50%
Initial conversion rate							0.480775
Initial conversion price per share (in dollars per share) | $ / shares							$ 52.000
Additional common shares (in shares)							2,363,103
Liquidation preference (in dollars per share) | $ / shares	$ 25.00						$ 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%						98.00%
Purchase price as a percentage of liquidation preference							100.00%
Number of consecutive trading days for calculating the average closing sale price of common shares	5 days
Common shares from conversion of preferred shares (in shares)							4,915,196
Series D | Fundamental change conversion right
Class of Stock [Line Items]
Preferred shares converted into common shares (in shares)	10,263,003
Distribution for the excess of the market value of common shares issued above carrying value of series D preferred shares redeemed | $	$ 16,200,000
Series E
Class of Stock [Line Items]
Preferred shares, of beneficial interest, shares outstanding (in shares)			11,000,000				11,000,000	11,000,000
Preferred shares dividend (in dollars per share) | $ / shares							$ 1.8125
Preferred shares, dividend yield							7.25%	7.25%
Liquidation preference (in dollars per share) | $ / shares							$ 25.00
Redemption price (in dollars per share) | $ / shares							$ 25.00
RMR
Class of Stock [Line Items]
Common shares issued (in shares)								90,135