Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Book value	$ 2,634,176	$ 2,422,891
Fair value	2,631,484	2,518,801
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	450,000
Payments/Payoffs	(232,916)
Adjustments to fair value	(104,401)
Long-term Debt, Gross	2,661,029	2,444,005
Debt Issuance Costs, Net	26,853	21,114
Fixed Rate Secured Debt [Member]
Book value	78,786	309,463
Fair value	81,481	325,753
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(230,618)
Adjustments to fair value	(13,654)
Variable Rate Secured Debt [Member]
Book value	2,200	2,500
Fair value	2,200	2,500
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	0
Payments/Payoffs	(300)
Adjustments to fair value	0
Unsecured Debt [Member]
Book value	2,580,043	2,132,042
Fair value	2,547,803	$ 2,190,548
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issuances	450,000
Payments/Payoffs	(1,998)
Adjustments to fair value	$ (90,747)