Shareholders' Equity (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended				12 Months Ended			2 Months Ended
	Feb. 28, 2015	Apr. 30, 2014	Jan. 31, 2013	Jul. 31, 2012	Dec. 31, 2014 property	Dec. 31, 2013	Dec. 31, 2012	Feb. 24, 2015	Jan. 12, 2015
Distributions
Distribution declared to common shareholders (in dollars per share)									$ 0.39
Distribution declared to common shareholders									$ 79,530
Distribution to common shareholders (in dollars per share)					$ 1.56	$ 1.56	$ 1.53
Dividend distributions as percentage of ordinary income					54.00%	66.19%	62.65%
Dividend distributions as percentage of return of capital					46.00%	21.30%	37.35%
Dividend distributions as percentage of capital gain					0.00%	3.10%	0.00%
Dividend distributions as percentage of unrecaptured Section 1250 gain					0.00%	9.41%	0.00%
Common shares issued in public offering	31,050,000	15,525,000	11,500,000	13,800,000
Net proceeds after underwriting discounts from issuance of common shares issued in public offering	$ 660,014	$ 322,807	$ 261,813	$ 287,052	$ 322,807	$ 261,813	$ 287,052
Number of properties pending acquisition					38
RMR
Distributions
Common shares issued in public offering					123,462	21,968	20,462
RMR | Subsequent event
Distributions
Common shares issued in public offering								25,263