Shareholders' Equity - Narrative (Details)					1 Months Ended	2 Months Ended	12 Months Ended			16 Months Ended
	May 15, 2016 USD ($) $ / shares shares	Mar. 17, 2016 USD ($)	Aug. 24, 2015 USD ($)	Mar. 25, 2014 USD ($) $ / shares shares	Jan. 31, 2014 USD ($) $ / shares	Sep. 30, 2016 USD ($) authorization	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2016 USD ($) $ / shares shares	Sep. 14, 2015 USD ($)
Class of Stock [Line Items]
Stock repurchase program, period in force		12 months	12 months
Redemption of shares | $							$ 275,000,000	$ 0	$ 0
Stock repurchase program, number of authorizations expired | authorization						2
Stock repurchase program, amount not utilized | $						$ 86,600,000
Distributions on common stock declared (in dollars per share) | $ / shares					$ 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	0	16,205,000
Preferred stock redemption premium | $							$ 9,609,000	$ 0	$ 0
Distribution percentage to shareholders consisting of an alternative minimum tax adjustment							96.31%	0.00%	15.41%
RMR
Class of Stock [Line Items]
Common shares issued (in shares) | shares									90,135
Common Shares
Class of Stock [Line Items]
Common shares issued to holders of preferred shares (in shares) | shares								144	10,412,499
Number of shares repurchased (in shares) | shares							2,491,675	3,410,300		5,901,975
Stock Repurchased and Retired During Period, Price Per Share | $ / shares							$ 27.68	$ 25.76		$ 26.57
Stock repurchase program, authorized amount | $			$ 100,000,000.0
Stock repurchase program, additional authorized amount | $		$ 150,000,000.0									$ 100,000,000.0
Redemption of shares | $							$ 69,000,000	$ 87,800,000		$ 156,800,000
Stock repurchase program, remaining authorized repurchase amount | $							$ 106,600,000			$ 106,600,000
Stock repurchase program, number of shares surrendered (in shares) | shares							31,025
Common shares from conversion of preferred shares (in shares) | shares							2,363,103			2,363,103
Common Shares | Fundamental change conversion right
Class of Stock [Line Items]
Preferred shares converted into common shares (in shares) | shares				10,411,779
Series D
Class of Stock [Line Items]
Number of shares repurchased (in shares) | shares							0	301	10,264,503
Redemption of shares | $							$ 0	$ 3,000	$ 249,004,000
Preferred shares, of beneficial interest, shares outstanding (in shares) | shares							4,915,196	4,915,196		4,915,196
Preferred shares dividend (in dollars per share) | $ / shares							$ 1.625
Preferred shares, dividend yield							6.50%	6.50%
Initial conversion rate							0.480775			0.480775
Initial conversion price per share (in dollars per share) | $ / shares							$ 52.00			$ 52.00
Additional common shares (in shares) | shares							2,363,103
Liquidation preference (in dollars per share) | $ / shares				$ 25.00			$ 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) | shares							4,915,196			4,915,196
Series D | Fundamental change conversion right
Class of Stock [Line Items]
Preferred shares converted into common shares (in shares) | 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]
Number of shares repurchased (in shares) | shares							11,000,000
Redemption of shares | $							$ 275,000,000
Preferred shares, of beneficial interest, shares outstanding (in shares) | shares							0	11,000,000		0
Preferred shares dividend (in dollars per share) | $ / shares							$ 1.8125
Preferred shares, dividend yield							0.00%	7.25%
Liquidation preference (in dollars per share) | $ / shares							$ 25.00			$ 25.00
Stock redeemed during period (in shares) | shares	11,000,000
Redemption price (in dollars per share) | $ / shares	$ 25.00
Preferred stock, redemption amount | $	$ 275,000,000
Preferred stock redemption premium | $							$ (9,609,000)