Schedule III - Real Estate and Accumulated Depreciation (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of Land and Buildings and Improvements
Balance at beginning of period	$ 3,461,251us-gaap_RealEstateGrossAtCarryingValue	$ 3,218,610us-gaap_RealEstateGrossAtCarryingValue	$ 2,908,626us-gaap_RealEstateGrossAtCarryingValue
Add: Acquisitions	573,936us-gaap_RealEstateOtherAcquisitions	219,169us-gaap_RealEstateOtherAcquisitions	238,731us-gaap_RealEstateOtherAcquisitions
Add: Improvements	59,485us-gaap_RealEstateImprovements	30,309us-gaap_RealEstateImprovements	72,149us-gaap_RealEstateImprovements
Less: Disposition of properties	(142,581)us-gaap_RealEstateCostOfRealEstateSold	(6,837)us-gaap_RealEstateCostOfRealEstateSold	0us-gaap_RealEstateCostOfRealEstateSold
Impairment charges on real estate assets owned at end of period	(9,200)us-gaap_ImpairmentOfRealEstate	0us-gaap_ImpairmentOfRealEstate	(896)us-gaap_ImpairmentOfRealEstate
Balance at end of period	3,711,887us-gaap_RealEstateGrossAtCarryingValue	3,461,251us-gaap_RealEstateGrossAtCarryingValue	3,218,610us-gaap_RealEstateGrossAtCarryingValue
Reconciliation of Accumulated Depreciation
Balance at beginning of period	(367,306)	(295,397)	(228,809)
Add: Depreciation for the period	(81,892)	(73,556)	(66,588)
Less: Disposition of properties	(24,383)	(1,647)	0
Balance at end of period	(382,266)	(367,306)	(295,397)
SEC Schedule III, Real Estate, Write-down or Reserve, Basis	-9200		-896
SEC Schedule III, Real Estate, Other Deductions	(231,004)us-gaap_RealEstateOtherDeductions
SEC Schedule III, Real Estate Accumulated Depreciation, Other Deductions	$ 42,549

The charge against earnings in the period to reduce the carrying amount of real property to fair value.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6391110&loc=d3e2921-110230

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- Definition

Amount of accumulated depreciation pertaining to real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 970

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-28.FN4)

-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule III)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

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Balance Type:	credit
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- Definition

Amount of other deductions to accumulated depreciation, excluding sales, for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

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- Definition

Amount of decrease to accumulated depreciation for real estate investments sold for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

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- Definition

Amount of real estate investments sold for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule III)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 970

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-28.4)

-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

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- Definition

Amount, before accumulated depreciation, of real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

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- Definition

Amount of increase in real estate investments related to improvements for entities with a substantial portion of business acquiring and holding investment real estate.

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Amount of increase in real estate investments from acquisitions other than through foreclosure for entities with a substantial portion of business acquiring and holding investment real estate.

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Amount of other deductions to real estate investments, excluding sales, for entities with a substantial portion of business acquiring and holding investment real estate.

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Description of reserve or write-down taken on real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule III)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 970

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-Section S99

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Amount of depreciation expense of real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

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