Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Book value	$ 3,341,739	$ 4,412,639
Fair value	3,487,842	4,778,176
Payoffs	(1,109,831)
Adjustments to fair value	(180,503)
Long Term Debt Including Amounts Classified as Held for Sale	3,341,739	4,453,403
Disposal Groups, Secured Debt Related to Assets Held for Sale	$ 0	$ 40,764
Fixed Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	6.55%	6.27%
Book value	$ 736,896	$ 979,842
Fair value	789,095	$ 1,065,301
Payoffs	(241,114)
Adjustments to fair value	$ (35,092)
Variable Rate Secured Debt [Member]
Debt, Weighted Average Interest Rate	0.03%	0.13%
Book value	$ 3,100	$ 3,400
Fair value	3,100	$ 3,400
Payoffs	(300)
Adjustments to fair value	$ 0
Unsecured Debt [Member]
Debt, Weighted Average Interest Rate	4.63%	5.22%
Book value	$ 2,530,743	$ 3,364,161
Fair value	2,624,795	$ 3,603,475
Payoffs	(833,417)
Adjustments to fair value	$ (145,263)
Unsecured Line of Credit DRLP [Member]
Debt, Weighted Average Interest Rate	1.41%	1.22%
Book value	$ 71,000	$ 106,000
Fair value	70,852	$ 106,000
Payoffs	(35,000)
Adjustments to fair value	$ (148)

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- Definition

Long Term Debt, Including Amounts Classified as Held for Sale

+ References

No definition available.

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- Definition

Weighted average interest rate of debt outstanding.

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

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20, 22
-Article 5

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