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Stock Options and Incentive Plans (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2013
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Prosperity Plus Savings Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Deferred compensation plan expense       $ 4.8 $ 4.3 $ 4.1
Prosperity Plus Savings Plan [Member] | Minimum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Percent of employee contribution matched by the Company       50.00%    
Prosperity Plus Savings Plan [Member] | Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Percent of employee contribution matched by the Company       100.00%    
Profit Sharing Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Deferred compensation plan expense       $ 2.4 1.5 1.6
Stock Options [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Contractual term of stock options, in years       20 years    
Length of time options are valid after retirement, in years       10 years    
Aggregate intrinsic value of vested options       $ 1.6    
Aggregate intrinsic value of options exercised       $ 2.0 0.7 3.1
Long Term Incentive Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares of stock authorized for payment of awards       288,219    
Shares issued for long term incentive plan 35,393 29,321 40,255      
Cash payments in connection with long term incentive plan $ 1.2 $ 1.1 $ 1.1      
Deferred compensation expense       $ 3.0 2.4 2.4
Vesting period       3 years    
Additional share based compensation expense expected to be recognized in next twelve months       $ 1.5    
Additional share based compensation expense expected to be recognized in two years       0.6    
Long Term Incentive Plan [Member] | Management [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares issued for long term incentive plan 19,571 15,910 34,988      
Cash payments in connection with long term incentive plan $ 1.5 $ 1.4 $ 2.0      
Deferred compensation expense       $ 3.4 2.7 2.3
Long Term Incentive Plan [Member] | Management [Member] | Minimum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Earned incentive compensation paid in shares of Class A Common Stock       40.00%    
Long Term Incentive Plan [Member] | Management [Member] | Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Earned incentive compensation paid in shares of Class A Common Stock       50.00%    
Restricted Stock [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Deferred compensation expense       $ 0.6 1.4 2.5
Vesting period       5 years    
Additional Restricted Stock Plan [Member] | Management [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Deferred compensation expense       $ 0.3 0.7 1.0
Phantom Stock Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Deferred compensation expense       2.6 $ 2.2 $ 1.5
Compensation cost not yet recognized       $ 6.6    
Vesting period       5 years    
Compensation cost recognition period       2 years