Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Book value	$ 4,208,547	$ 4,254,376
Fair value	4,540,731	4,503,707
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	52,000
Payments/Payoffs	(94,592)
Adjustments to fair value	79,616
Fixed Rate Secured Debt [Member]
Book value	1,000,451	1,081,035
Fair value	1,090,414	1,145,717
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(77,347)
Adjustments to fair value	22,044
Variable Rate Secured Debt [Member]
Book value	3,400	19,089
Fair value	3,400	19,089
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(15,689)
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	3,064,696	3,066,252
Fair value	3,306,438	3,250,518
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(1,556)
Adjustments to fair value	57,476
Line of Credit [Member]
Book value	140,000	88,000
Fair value	140,479	88,383
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	52,000
Payments/Payoffs	0
Adjustments to fair value	$ 96