Property: (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Abstract]
Land	$ 2,242,291us-gaap_Land	$ 1,707,005us-gaap_Land
Buildings and improvements	9,479,337	6,555,212
Tenant improvements	600,436us-gaap_TenantImprovements	537,754us-gaap_TenantImprovements
Equipment and furnishings	152,554us-gaap_FixturesAndEquipmentGross	152,198us-gaap_FixturesAndEquipmentGross
Construction in progress	303,264us-gaap_DevelopmentInProcess	229,169us-gaap_DevelopmentInProcess
Total	12,777,882	9,181,338
Less accumulated depreciation	(1,709,992)	(1,559,572)
Property, net	11,067,890us-gaap_RealEstateInvestmentPropertyNet	7,621,766us-gaap_RealEstateInvestmentPropertyNet
Depreciation expense	$ 289,178us-gaap_DepreciationNonproduction	$ 269,790us-gaap_DepreciationNonproduction	$ 216,447us-gaap_DepreciationNonproduction

The expense recognized in the current period that allocates the cost of nonproduction tangible assets over their useful lives.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

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

+ Details

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

- Definition

The current amount of expenditures for a real estate project that has not yet been completed.

+ References

+ Details

Name:	us-gaap_DevelopmentInProcess
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before accumulated depreciation of fixtures and equipment. Includes, but is not limited to, machinery, equipment, and engines.

+ References

No definition available.

+ Details

Name:	us-gaap_FixturesAndEquipmentGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Aggregate of the carrying amounts as of the balance sheet date of investments in building and building improvements.

+ References

+ Details

Name:	us-gaap_InvestmentBuildingAndBuildingImprovements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before accumulated depletion of real estate held for productive use, excluding land held for sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section 45

-Paragraph 4

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6812-107765

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.13)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_Land
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_PropertyPlantAndEquipmentAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The cumulative amount of depreciation for real estate property held for investment purposes.

+ 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(3))

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 7

+ Details

Name:	us-gaap_RealEstateInvestmentPropertyAccumulatedDepreciation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

+ Details

Name:	us-gaap_RealEstateInvestmentPropertyAtCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of real estate investment property, net of accumulated depreciation, 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

+ Details

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

- Definition

Carrying amount as of the balance sheet date of improvements having a life longer than one year that were made for the benefit of one or more tenants.

+ References

+ Details

Name:	us-gaap_TenantImprovements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant