Credit Risk and Fair Value of Financial Instruments Level 4 (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage notes and other debt, maturities through 2047	$ 36,394	$ 37,991
Total fair value of debt instruments	1,827,160	1,789,056
February 2013 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
7.875% Debentures due February 2013	5,173	9,092
October 2014 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
7.875% Debentures due February 2013	199,665	194,244
April 2015 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
7.875% Debentures due February 2013	152,859	161,968
April 2016 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
7.875% Debentures due February 2013	222,380	216,653
June 2017 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
7.875% Debentures due February 2013	317,125	302,375
October 2018 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
7.875% Debentures due February 2013	275,313	262,500
May 2019 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
7.875% Debentures due February 2013	263,125	251,250
November 2021 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
7.875% Debentures due February 2013	162,750	158,063
April 2027 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
7.875% Debentures due February 2013	$ 192,376	$ 194,920