Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Book value	$ 4,133,885	$ 4,254,376
Fair value	4,495,978	4,503,707
Payments/Payoffs	(117,927)
Adjustments to fair value	110,198
Fixed Rate Secured Debt [Member]
Book value	1,004,327	1,081,035
Fair value	1,098,560	1,145,717
Payments/Payoffs	(74,144)
Adjustments to fair value	26,987
Variable Rate Secured Debt [Member]
Book value	4,335	19,089
Fair value	4,335	19,089
Payments/Payoffs	(14,754)
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	3,065,223	3,066,252
Fair value	3,332,841	3,250,518
Payments/Payoffs	(1,029)
Adjustments to fair value	83,352
Line of Credit [Member]
Book value	60,000	88,000
Fair value	60,242	88,383
Payments/Payoffs	(28,000)
Adjustments to fair value	$ (141)