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	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Long Term Debt Maturities Repayments Of Principal	$ 2,400,130
Derivative, Notional Amount	428,610	428,610
Derivative, Fair Value, Net	12,383	14,210
3.623% Interest Rate Swap Maturing February 1, 2019 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	200,000	200,000
Derivative, Fair Value, Net	7,761	8,859
Derivative, Fixed Interest Rate	3.623%
2.703% Interest Rate Swap Maturing February 1, 2016 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	77,000	77,000
Derivative, Fair Value, Net	1,229	1,343
Derivative, Fixed Interest Rate	2.703%
2.470% Interest Rate Swap Maturing February 1, 2015 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	50,000	50,000
Derivative, Fair Value, Net	404	458
Derivative, Fixed Interest Rate	2.47%
2.513% Interest Rate Swap Maturing May 1, 2015 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	23,000	23,000
Derivative, Fair Value, Net	221	245
Derivative, Fixed Interest Rate	2.513%
2.750% Interest Rate Swap Maturing September 30, 2017 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	27,062	27,062
Derivative, Fair Value, Net	825	914
Derivative, Fixed Interest Rate	2.75%
3.300% Interest Rate Swap Maturing January 30, 2021 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	25,774	25,774
Derivative, Fair Value, Net	1,003	1,262
Derivative, Fixed Interest Rate	3.30%
3.090% Interest Rate Swap Maturing October 30, 2019 [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	25,774	25,774
Derivative, Fair Value, Net	$ 940	$ 1,129
Derivative, Fixed Interest Rate	3.09%