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Real Estate Properties (Details 2) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2012
government
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
government
Dec. 31, 2011
Dec. 31, 2010
Concentration risk relating to future minimum lease payments                      
Number of state governments 10               10    
Summarized income statement information for properties sold                      
Rental income $ 55,850 $ 53,593 $ 49,808 $ 49,997 $ 51,106 $ 45,243 $ 41,451 $ 38,579 $ 209,247 $ 176,379 $ 116,345
Real estate taxes                 (23,202) (19,136) (12,093)
Utility expenses                 (16,418) (14,805) (8,929)
Other operating expenses                 (38,155) (31,234) (19,746)
Depreciation and amortization                 (50,590) (39,426) (24,063)
Loss on asset impairment                 (494)    
Acquisition related costs                 (1,614) (3,504) (5,582)
General and administrative                 (12,202) (10,746) (7,007)
Income (loss) from discontinued operations                 (400) 486 72
Discontinued operations
                     
Summarized income statement information for properties sold                      
Rental income                 1,829 2,571 874
Real estate taxes                 (211) (209) (84)
Utility expenses                 (392) (511) (135)
Other operating expenses                 (403) (550) (191)
Depreciation and amortization                 (576) (663) (176)
Loss on asset impairment                 (494)    
Acquisition related costs                     (168)
General and administrative                 (153) (152) (48)
Income (loss) from discontinued operations                 $ (400) $ 486 $ 72
Future minimum lease payments | Tenant concentration | State government
                     
Concentration risk relating to future minimum lease payments                      
Concentration risk, percentage                 7.10%    
Number of state governments 12               12    
Future minimum lease payments | Tenant concentration | Government
                     
Concentration risk relating to future minimum lease payments                      
Concentration risk, percentage                 3.30%    
Concentration risk, percentage for termination right exercisable in 2013 3.80%               3.80%    
Concentration risk, percentage for termination right exercisable in 2014 3.60%               3.60%    
Concentration risk, percentage for termination right exercisable in 2015 0.40%               0.40%    
Concentration risk, percentage for termination right exercisable in 2016 12.60%               12.60%    
Concentration risk, percentage for termination right exercisable in 2017 5.60%               5.60%    
Concentration risk, percentage for termination right exercisable in 2018 2.00%               2.00%    
Concentration risk, percentage for termination right exercisable in 2019 7.70%               7.70%    
Concentration risk, percentage for termination right exercisable in 2020 1.60%               1.60%