Risk Management and Use of Financial Instruments Risk Management and Use of Financial Instruments (Fair Values of Derivative Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	$ 51,548	$ 51,548
Derivative, Fair Value, Net	175	(2,391)
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	377,062	377,062
Derivative, Fair Value, Net	(3,089)	(11,819)
3.300% Interest Rate Swap Maturing January 30, 2021 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	25,774	25,774
Derivative, Fixed Interest Rate	3.30%
Derivative, Fair Value, Net	154	(1,262)
3.090% Interest Rate Swap Maturing October 30, 2019 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	25,774	25,774
Derivative, Fixed Interest Rate	3.09%
Derivative, Fair Value, Net	21	(1,129)
3.623% Interest Rate Swap Maturing February 1, 2019 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	200,000	200,000
Derivative, Fixed Interest Rate	3.623%
Derivative, Fair Value, Net	(1,506)	(8,859)
2.703% Interest Rate Swap Maturing February 1, 2016 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	77,000	77,000
Derivative, Fixed Interest Rate	2.703%
Derivative, Fair Value, Net	(798)	(1,343)
2.470% Interest Rate Swap Maturing February 1, 2015 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	50,000	50,000
Derivative, Fixed Interest Rate	2.47%
Derivative, Fair Value, Net	(328)	(458)
2.513% Interest Rate Swap Maturing May 1, 2015 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	23,000	23,000
Derivative, Fixed Interest Rate	2.513%
Derivative, Fair Value, Net	(174)	(245)
2.750% Interest Rate Swap Maturing September 30, 2017 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	27,062	27,062
Derivative, Fixed Interest Rate	2.75%
Derivative, Fair Value, Net	$ (283)	$ (914)