Real Estate and Accumulatied Deprecitation Reconcilation Real estate and accumulated depreciation (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
SEC Schedule III, Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Real Estate, Gross, Starting Balance	$ 18,953,601us-gaap_RealEstateGrossAtCarryingValue	$ 14,869,887us-gaap_RealEstateGrossAtCarryingValue	$ 13,363,113us-gaap_RealEstateGrossAtCarryingValue
Additions to/improvements of real estate	594,296us-gaap_RealEstateImprovements	4,410,622us-gaap_RealEstateImprovements	1,602,583us-gaap_RealEstateImprovements
Assets sold/written-off	(339,480)us-gaap_RealEstateCostOfRealEstateSold	(326,908)us-gaap_RealEstateCostOfRealEstateSold	(95,809)us-gaap_RealEstateCostOfRealEstateSold
Real Estate, Gross, Ending Balance	19,208,417us-gaap_RealEstateGrossAtCarryingValue	18,953,601us-gaap_RealEstateGrossAtCarryingValue	14,869,887us-gaap_RealEstateGrossAtCarryingValue
SEC Schedule III, Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Real Estate Accumulated Depreciation, Beginning Balance	3,145,701	2,919,479	2,626,324
Depreciation expense	456,176	419,908	367,625
Assets sold/written-off	(71,899)	(193,686)	(74,470)
Real Estate Accumulated Depreciation, Ending Balance	$ 3,529,978	$ 3,145,701	$ 2,919,479

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

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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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

+ Details

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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.

+ References

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- Definition

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

+ References

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