Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Disposal Groups, Secured Debt Related to Assets Held for Sale	$ 41,697	$ 40,764
Book value	4,444,368us-gaap_LongTermDebt	4,412,639us-gaap_LongTermDebt
Fair value	4,834,826	4,778,176
Long Term Debt Including Amounts Classified as Held for Sale	4,486,065	4,453,403
Issuance and assumptions	347,000
Payments/Payoffs	(313,695)
Adjustments to fair value	23,345
Fixed Rate Secured Debt [Member]
Book value	916,048	979,842
Fair value	999,944	1,065,301
Issuance and assumptions	0
Payments/Payoffs	(63,151)
Adjustments to fair value	(2,206)
Variable Rate Secured Debt [Member]
Book value	3,400	3,400
Fair value	3,400	3,400
Issuance and assumptions	0
Payments/Payoffs	0
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	3,113,617	3,364,161
Fair value	3,378,482	3,603,475
Issuance and assumptions	0
Payments/Payoffs	(250,544)
Adjustments to fair value	25,551
Unsecured Line of Credit DRLP [Member]
Book value	453,000	106,000
Fair value	453,000	106,000
Issuance and assumptions	347,000
Payments/Payoffs	0
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 issuances 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

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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

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-Publisher FASB

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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.

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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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

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