Risk Management and Use of Financial Instruments (Fair Values of Derivative Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Notional Amount	$ 428,610		$ 452,836
Fair value	14,210		3,886
Long Term Debt Maturities Repayments Of Principal	2,471,895
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(2,985)		0		0
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	8,859	[1]	2,820	[1]
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | 2.703% Interest Rate Swap Maturing February 1, 2016 [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	77,000	[1],[2]	127,000	[1],[2]
Derivative, Fixed Interest Rate	2.703%	[1],[2]
Fair value	1,343	[1],[2]	340	[1],[2]
Derivative, Notional Amount Repaid	50,000
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	458	[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	245	[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	914	[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	1,262	[1]	390	[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	1,129	[1]	0	[1]
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | 2.780% and 2.910% Interest Rate Swaps Repaid [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount Repaid	100,000
Four-Year Term Loan - Swapped to fixed [Member] | Unsecured Debt [Member]
Derivatives, Fair Value [Line Items]
Long Term Debt Maturities Repayments Of Principal	150,000
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 3,000

[1] Hedging unsecured variable rate debt.

[2] On December 31, 2012, the Company repaid the entire $150.0 million swapped to fixed portion of its previously mentioned four-year loan (please refer to Note 7), consisting of $50.0 million interest rate swaps maturing February 1, 2016 (shown above), and $100.0 million of interest rate swaps entered into on January 6, 2012. In connection with the repayment, the Company terminated associated hedging contracts, incurring a $3.0 million charge upon termination.