Schedule III - Real Estate and Accumulated Depreciation (Aggregate Costs for Federal Income Tax Purposes and Ownership Percentage) (Details) (USD $) In Billions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Real Estate and Accumulated Depreciation [Line Items]
Tax Basis of Assets, Cost for Income Tax Purposes	$ 3.7us-gaap_RealEstateFederalIncomeTaxBasis	$ 4.0us-gaap_RealEstateFederalIncomeTaxBasis
3141 Fairview Park Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Equity Method Investment, Ownership Percentage	50.00%

Cost basis of real estate investments for federal income tax purposes 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 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.FN6)

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

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