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Equity-Based Compensation Plans (Details 2) (USD $)
3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Performance Shares 2010 Award [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Requisite Service Period       3 years
Percent allocated to performance year one 33.00%      
Percent allocated to performance year two 33.00%      
Percent allocated to performance year three 33.00%      
Performance percent target achieved year one       150.00%
Performance percent target achieved year two     150.00%  
Performance percent target achieved year three   0.00%    
Summary of performance share unit        
Number of Shares, Beginning balance 33,700      
Number of Performance Share Units, Cancelled 0      
Number of Performance Share Units, Issued (33,700)      
Number of Share Units, Ending balance 0 33,700    
Performance Shares 2011 Award [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Requisite Service Period     3 years  
Percent allocated to performance year one 100.00%      
Performance percent target achieved year one     200.00%  
Summary of performance share unit        
Number of Shares, Beginning balance 166,750      
Number of Performance Share Units, Cancelled (600)      
Number of Performance Share Units, Issued 0      
Number of Share Units, Ending balance 166,150      
Performance Shares [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Requisite Service Period 3 years      
Summary of performance share unit        
Number of Shares, Beginning balance 200,450      
Weighted Average Exercise Price, Beginning balance 26.31      
Number of Performance Share Units, Cancelled (600)      
Weighted Average Exercise Price, Cancelled 27.00      
Number of Performance Share Units, Issued (33,700)      
Weighted Average Exercise Price, Issued 25.61      
Number of Share Units, Ending balance 166,150      
Weighted Average Exercise Price, Ending balance 26.58      
Minimum [Member] | Performance Shares 2010 Award [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share Based Compensation Arrangement by Share Based Payment Award Equity Instrument Other Than Option Stock Issuance Percentage Multiplier       0.00%
Minimum [Member] | Performance Shares 2011 Award [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share Based Compensation Arrangement by Share Based Payment Award Equity Instrument Other Than Option Stock Issuance Percentage Multiplier     0.00%  
Maximum [Member] | Performance Shares 2010 Award [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share Based Compensation Arrangement by Share Based Payment Award Equity Instrument Other Than Option Stock Issuance Percentage Multiplier       150.00%
Maximum [Member] | Performance Shares 2011 Award [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share Based Compensation Arrangement by Share Based Payment Award Equity Instrument Other Than Option Stock Issuance Percentage Multiplier     200.00%