Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2015	Dec. 31, 2014
Disposal Groups, Secured Debt Related to Assets Held for Sale	$ 0	$ 40,764
Book value	3,460,743	4,412,639
Fair value	3,658,818	4,778,176
Long Term Debt Including Amounts Classified as Held for Sale	3,460,743	4,453,403
Payments/Payoffs	(991,205)
Adjustments to fair value	(128,153)
Fixed Rate Secured Debt [Member]
Book value	775,469	979,842
Fair value	845,979	1,065,301
Payments/Payoffs	(202,918)
Adjustments to fair value	(16,404)
Variable Rate Secured Debt [Member]
Book value	3,400	3,400
Fair value	3,400	3,400
Payments/Payoffs	0
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	2,681,874	3,364,161
Fair value	2,809,439	3,603,475
Payments/Payoffs	(682,287)
Adjustments to fair value	(111,749)
Unsecured Line of Credit DRLP [Member]
Book value	0	106,000
Fair value	0	$ 106,000
Payments/Payoffs	(106,000)
Adjustments to fair value	$ 0

Disposal Groups, Secured Debt Related to Assets Held for Sale

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- Definition

Long Term Debt, Including Amounts Classified as Held for Sale

+ References

No definition available.

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- Definition

Amount of increase (decrease) of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

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

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20, 22

-Article 5

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