Real Estate (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Real Estate Investment Property, at Cost [Abstract]
Land and Land Improvements	$ 1,189,760,000us-gaap_LandAndLandImprovements	$ 1,137,897,000us-gaap_LandAndLandImprovements
Building and improvements	5,343,908,000	5,129,702,000
Total Real Estate Investment Property, at Cost	6,533,668,000	6,267,599,000
Accumulated Depreciation	1,182,569,000	1,051,593,000
Depreciation	179,100,000us-gaap_Depreciation	162,500,000us-gaap_Depreciation	140,600,000us-gaap_Depreciation
Industrial Property [Member]
Real Estate Investment Property, at Cost [Abstract]
Land and Land Improvements	773,874,000	707,951,000
Building and improvements	3,513,534,000	3,236,596,000
Total Real Estate Investment Property, at Cost	4,287,408,000	3,944,547,000
Accumulated Depreciation	620,967,000	514,073,000
Office Building [Member]
Real Estate Investment Property, at Cost [Abstract]
Land and Land Improvements	415,886,000	429,946,000
Building and improvements	1,830,374,000	1,893,106,000
Total Real Estate Investment Property, at Cost	2,246,260,000	2,323,052,000
Accumulated Depreciation	$ 561,602,000	$ 537,520,000

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

Reference 1: 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

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Period Type:	duration

- Definition

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

+ References

+ Details

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- Definition

Amount before accumulated deprecation and depletion of real estate held for productive use and additions or improvements to real estate held for productive use, examples include, but are not limited to, walkways, driveways, fences, and parking lots. Excludes 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 S99

-Paragraph 1

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section 45

-Paragraph 4

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

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant

- 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

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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

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