Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Book value	$ 4,435,211	$ 4,446,170
Fair value	4,724,586	4,891,881
Issuances and assumptions	605,571
Payments/Payoffs	(613,559)
Adjustments to fair value	(159,307)
Fixed Rate Secured Debt [Member]
Book value	1,139,262	1,149,541
Fair value	1,220,368	1,251,477
Issuances and assumptions	103,638
Payments/Payoffs	(110,946)
Adjustments to fair value	(23,801)
Variable Rate Secured Debt [Member]
Book value	19,194	18,412
Fair value	19,195	18,386
Issuances and assumptions	1,933
Payments/Payoffs	(1,151)
Adjustments to fair value	27
Unsecured Debt [Member]
Book value	3,066,755	2,993,217
Fair value	3,273,988	3,336,386
Issuances and assumptions	500,000
Payments/Payoffs	(426,462)
Adjustments to fair value	(135,936)
Line of Credit [Member]
Book value	210,000	285,000
Fair value	211,035	285,632
Issuances and assumptions	0
Payments/Payoffs	(75,000)
Adjustments to fair value	$ 403