Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Book value	$ 4,396,777	$ 4,446,170
Fair value	4,707,976	4,891,881
Issuances and assumptions	605,571
Payments/Payoffs	(653,316)
Adjustments to fair value	(136,160)
Fixed Rate Secured Debt [Member]
Book value	1,221,348	1,149,541
Fair value	1,308,795	1,251,477
Issuances and assumptions	103,638
Payments/Payoffs	(30,349)
Adjustments to fair value	(15,971)
Variable Rate Secured Debt [Member]
Book value	20,179	18,412
Fair value	20,149	18,386
Issuances and assumptions	1,933
Payments/Payoffs	0
Adjustments to fair value	(170)
Unsecured Debt [Member]
Book value	3,067,250	2,993,217
Fair value	3,291,032	3,336,386
Issuances and assumptions	500,000
Payments/Payoffs	(425,967)
Adjustments to fair value	(119,387)
Line of Credit [Member]
Book value	88,000	285,000
Fair value	88,000	285,632
Issuances and assumptions	0
Payments/Payoffs	(197,000)
Adjustments to fair value	$ (632)