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Stock Options and Incentive Plans (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
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       $ 1.3    
Aggregate intrinsic value of options exercised       $ 1.1 $ 0.5 $ 2.0
Long Term Incentive Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares of stock authorized for payment of awards       190,616    
Shares issued for long term incentive plan 25,899 26,146 35,393      
Cash payments in connection with long term incentive plan $ 1.0 $ 0.8 $ 1.2      
Deferred compensation expense       $ 2.6 2.7 3.0
Vesting period       3 years    
Additional share based compensation expense expected to be recognized in next twelve months       $ 1.2    
Additional share based compensation expense expected to be recognized in two years       0.4    
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 18,784 26,774 19,571      
Cash payments in connection with long term incentive plan $ 1.9 $ 1.9 $ 1.5      
Deferred compensation expense       2.6 3.3 3.4
Phantom Stock Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Deferred compensation expense       4.9 3.8 2.6
Compensation cost not yet recognized       $ 10.0    
Vesting period       5 years    
Compensation cost recognition period       2 years    
Minimum [Member] | Long Term Incentive Plan [Member] | Management [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%    
Maximum [Member] | Long Term Incentive Plan [Member] | Management [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%    
Pension Plans [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Deferred compensation plan expense       $ 5.9 5.5 4.8
Pension Plans [Member] | Minimum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Percent of employee contribution matched by the Company       50.00%    
Pension Plans [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       $ 2.6 $ 2.9 $ 2.4