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Income Taxes - Narrative (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Income Taxes:    
Income tax expense for income and the portion of earnings retained based on 162(m) limitations $ 3.9 $ 14.7
Years federal and state tax returns remain open for examination 2009, 2010 and 2011  
Real Estate Investment Trust
   
Income Taxes:    
Income tax expense for income and the portion of earnings retained based on 162(m) limitations $ 1.9 $ 1.8