Indebtedness - Amortization and Maturity Schedule (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
2012	$ 265,145
2013	9,092
2014	207,649
2015	504,380
2016	485,386
2017	301,210
2018	100,000
2019	0
2020	350,000
Mortgages Principal Amortization	24,375
Mortgages Principal Maturities	266,444
Unsecured Debt	1,792,643	2,039,143
Line of Credit Facility, Amount Outstanding	139,400	0
Long-term Debt	2,222,862
Long-term Debt, Weighted Average Interest Rate	5.58%
Principal Amortization, Mortgages [Member]
Debt Instrument [Line Items]
2012	4,931
2013	4,582
2014	4,965
2015	4,511
2016	3,068
2017	2,318
2018	0
2019	0
2020	0
Principal Maturities, Mortgages [Member]
Debt Instrument [Line Items]
2012	30,114
2013	4,510
2014	2,684
2015	44,469
2016	182,318
2017	2,349
2018	0
2019	0
2020	0
Unsecured Debt [Member]
Debt Instrument [Line Items]
2012	230,100
2013	0
2014	200,000
2015	316,000
2016	300,000
2017	296,543
2018	100,000
2019	0
2020	350,000
Line of Credit [Member]
Debt Instrument [Line Items]
2012	0
2013	0
2014	0
2015	139,400
2016	0
2017	0
2018	0
2019	0
2020	$ 0
2012
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	6.47%
2013
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	5.73%
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	4.15%
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	6.61%
2018
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	7.50%
2019
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	0.00%
2020
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	4.75%