Concentration of Credit Risk (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Concentration of Credit Risk.
Investment	$ 6,238,611	$ 5,263,625
Revenue	526,703us-gaap_RealEstateRevenueNet	459,380us-gaap_RealEstateRevenueNet	450,769us-gaap_RealEstateRevenueNet
Five Star \| Senior living communities
Concentration of Credit Risk.
Number of properties acquired, referred to as managed properties	46
Investment \| Rents from significant lessee
Concentration of Credit Risk.
Investment	6,238,611	5,263,625
% of Total (as a percent)	100.00%	100.00%
Investment \| Rents from significant lessee \| Five Star
Concentration of Credit Risk.
Investment	2,125,517	2,099,713
% of Total (as a percent)	34.00%	40.00%
Investment \| Rents from significant lessee \| All others
Concentration of Credit Risk.
Investment	4,113,094	3,163,912
% of Total (as a percent)	66.00%	60.00%
Rental income \| Rents from significant lessee
Concentration of Credit Risk.
Revenue	526,703	459,380
% of Total (as a percent)	100.00%	100.00%
Rental income \| Rents from significant lessee \| Five Star
Concentration of Credit Risk.
Revenue	196,269	203,719
% of Total (as a percent)	37.00%	44.00%
Rental income \| Rents from significant lessee \| All others
Concentration of Credit Risk.
Revenue	$ 330,434	$ 255,661
% of Total (as a percent)	63.00%	56.00%

Represents the number of properties acquired or agreed to be acquired by the entity during the period, which are referred by the entity as managed properties.

No definition available.

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 21

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 18

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592

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- Definition

Amount of real estate investment property which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; and (7) other real estate investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.1(d))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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- Definition

The aggregate revenue from real estate operations during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1(e))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

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