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Stock-Based Compensation (Details Textual 1) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Allocated Share-based Compensation Expense $ 1.4 $ 1.2
FFO RSUs [Member]
   
Share-based Compensation [Abstract]    
Number of restricted stock units granted 100,571  
Payout range minimum 0.00%  
Payout range maximum 200.00%  
Performance period January 1, 2012 to December 31, 2014  
Share-based Compensation Arrangement by Share-based Payment Award, Vesting Date Feb. 15, 2015  
Share-based Payment Arrangement by Award, Calculation of Payout per Unit Average closing price on each day of trading during the 30-day vesting period ending December 31, 2014  
SNL RSUs [Member]
   
Share-based Compensation [Abstract]    
Number of restricted stock units granted 159,640  
Payout range minimum 0.00%  
Payout range maximum 200.00%  
Performance period January 1, 2012 to December 31, 2014  
Share-based Compensation Arrangement by Share-based Payment Award, Vesting Date Feb. 15, 2015  
Share-based Payment Arrangement by Award, Calculation of Payout per Unit Average closing price on each day of trading during the 30-day vesting period ending December 31, 2014  
Restricted Stock [Member]
   
Share-based Compensation [Abstract]    
Number of restricted stock units granted 255,328  
Vesting period of restricted stock units 3 years  
Chief Executive Officer [Member] | SNL RSUs [Member]
   
Share-based Compensation [Abstract]    
Number of restricted stock units granted 281,532  
Payout range minimum 0.00%  
Payout range maximum 150.00%  
Performance period January 1, 2012 to December 31, 2016  
Interim Performance Measurement Dates interim performance measurement dates at each of the third, fourth and fifth anniversaries  
Share-based Payment Arrangement by Award, Calculation of Payout per Unit average closing price on each trading day during the 30-day period ending with such date  
Chief Executive Officer [Member] | Restricted Stock [Member]
   
Share-based Compensation [Abstract]    
Number of restricted stock units granted 208,333  
Vesting period of restricted stock units 3 years