Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Book value	$ 4,323,210	$ 4,254,376
Fair value	4,624,645	4,503,707
Issuances and assumptions	92,000
Payments/Payoffs	(21,982)
Adjustments to fair value	50,920
Fixed Rate Secured Debt [Member]
Book value	1,058,485	1,081,035
Fair value	1,135,596	1,145,717
Issuances and assumptions	0
Payments/Payoffs	(21,365)
Adjustments to fair value	11,244
Variable Rate Secured Debt [Member]
Book value	18,983	19,089
Fair value	18,983	19,089
Issuances and assumptions	0
Payments/Payoffs	(106)
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	3,065,742	3,066,252
Fair value	3,289,215	3,250,518
Issuances and assumptions	0
Payments/Payoffs	(511)
Adjustments to fair value	39,208
Line of Credit [Member]
Book value	180,000	88,000
Fair value	180,851	88,383
Issuances and assumptions	92,000
Payments/Payoffs	0
Adjustments to fair value	$ 468