Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Book value	$ 4,453,403us-gaap_LongTermDebt	$ 4,254,376us-gaap_LongTermDebt
Fair value	4,778,176	4,503,707
Issuances and assumptions	318,000
Payoffs	(114,969)
Adjustments to fair value	71,438
Fixed Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	6.27%	6.23%
Book value	979,842	1,081,035
Fair value	1,065,301	1,145,717
Issuances and assumptions	0
Payoffs	(97,188)
Adjustments to fair value	16,772
Variable Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	0.13%	2.11%
Book value	3,400	19,089
Fair value	3,400	19,089
Issuances and assumptions	0
Payoffs	(15,689)
Adjustments to fair value	0
Unsecured Debt [Member]
Debt, Weighted Average Interest Rate	5.22%	5.36%
Book value	3,364,161	3,066,252
Fair value	3,603,475	3,250,518
Issuances and assumptions	300,000
Payoffs	(2,092)
Adjustments to fair value	55,049
Unsecured Line of Credit DRLP [Member]
Debt, Weighted Average Interest Rate	1.22%	1.42%
Book value	106,000	88,000
Fair value	106,000	88,383
Issuances and assumptions	18,000
Payoffs	0
Adjustments to fair value	$ (383)

Weighted average interest rate of debt outstanding.

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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.

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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.

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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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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.

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