Summary of Significant Accounting Policies (Accrued Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Accrued liabilities
Property taxes	$ 10,430	$ 11,245
Accrued interest	9,205	10,074
Accrued payroll	9,953	10,225
Other accrued liabilities	5,478	4,690
Total accrued liabilities	$ 35,066	$ 36,234