Indebtedness - Amortization and Maturity Schedule (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
2013	$ 9,944
2014	209,188
2015	458,576
2016	487,282
2017	302,724
2018	101,821
2019	4,932
2020	354,744
2021	1,654
2022 and thereafter	726,533
Unsecured Debt	2,262,543	1,792,643
Line of Credit Facility, Amount Outstanding	92,000	139,400
Long-term Debt	2,657,398
Long-term Debt, Weighted Average Interest Rate	5.06%
Principal Amortization, Mortgages [Member]
Debt Instrument [Line Items]
2013	5,438
2014	6,492
2015	6,107
2016	4,964
2017	3,832
2018	1,821
2019	1,811
2020	1,749
2021	1,654
2022 and thereafter	24,586
Mortgages Principal Amortization	58,454
Principal Maturities, Mortgages [Member]
Debt Instrument [Line Items]
2013	4,506
2014	2,696
2015	44,469
2016	182,318
2017	2,349
2018	0
2019	3,121
2020	2,995
2021	0
2022 and thereafter	1,947
Mortgages Principal Maturities	244,401
Unsecured Debt [Member]
Debt Instrument [Line Items]
2013	0
2014	200,000
2015	316,000
2016	300,000
2017	296,543
2018	100,000
2019	0
2020	350,000
2021	0
2022 and thereafter	700,000
Unsecured Debt	2,262,543
Line of Credit [Member]
Debt Instrument [Line Items]
2013	0
2014	0
2015	92,000
2016	0
2017	0
2018	0
2019	0
2020	0
2021	0
2022 and thereafter	0
Line of Credit Facility, Amount Outstanding	$ 92,000
2013
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	5.59%
2014
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	5.66%
2015
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	4.35%
2016
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	6.10%
2017
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	6.60%
2018
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	7.45%
2019
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	3.55%
2020
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	4.74%
2021
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	4.76%
2022 and thereafter
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	3.84%

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Amount borrowed under the credit facility as of the balance sheet date.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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