Risk Management and Use of Financial Instruments (Fair Values of Derivative Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Notional Amount	$ 578,610		$ 452,836
Fair value	18,741		3,886
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | 3.623% Interest Rate Swap Maturing February 1, 2019 [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	200,000	[1]	200,000	[1]
Derivative, Fixed Interest Rate	3.623%	[1]
Fair value	9,523	[1]	2,820	[1]
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | 2.702% Interest Rate Swap Maturing February 1, 2016 [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	127,000	[1]	127,000	[1]
Derivative, Fixed Interest Rate	2.702%	[1]
Fair value	2,385	[1]	340	[1]
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | 2.470% Interest Rate Swap Maturing February 1, 2015 [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	50,000	[1]	50,000	[1]
Derivative, Fixed Interest Rate	2.47%	[1]
Fair value	509	[1]	65	[1]
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | 2.513% Interest Rate Swap Maturing May 1, 2015 [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	23,000	[1]	23,000	[1]
Derivative, Fixed Interest Rate	2.513%	[1]
Fair value	271	[1]	27	[1]
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | 2.750% Interest Rate Swap Maturing September 30, 2017 [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	27,062	[1]	27,062	[1]
Derivative, Fixed Interest Rate	2.75%	[1]
Fair value	993	[1]	244	[1]
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | 3.300% Interest Rate Swap Maturing January 30, 2021 [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	25,774	[1]	25,774	[1]
Derivative, Fixed Interest Rate	3.30%	[1]
Fair value	1,408	[1]	390	[1]
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | 2.780% Interest Rate Swap Maturing August 1, 2016 [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	50,000	[1]	0	[1]
Derivative, Fixed Interest Rate	2.78%	[1]
Fair value	1,095	[1]	0	[1]
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | 2.910% Interest Rate Swap Maturing February 1, 2017 [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	50,000	[1]	0	[1]
Derivative, Fixed Interest Rate	2.91%	[1]
Fair value	1,327	[1]	0	[1]
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | 3.090% Interest Rate Swap Maturing October 30, 2019 [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	25,774	[1]	0	[1]
Derivative, Fixed Interest Rate	3.09%	[1]
Fair value	$ 1,230	[1]	$ 0	[1]

[1] Hedging unsecured variable rate debt.