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Equity Incentive Programs (Equity Incentive Programs and Compensation Expense) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Aggregate shares of common stock reserved (in shares) 10,871,652   10,871,652  
Stock option exercise price floor of fair market value of class A common stock (percent)     100.00%  
Shares available for issuance (in shares) 1,563,344   1,563,344  
Stock-based compensation $ 3.2 $ 3.3 $ 12.3 $ 13.0
Estimated future compensation expense 19.3   $ 19.3  
Restricted Stock and Restricted Stock Units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period     3 years  
Estimated future compensation expense 19.3   $ 19.3  
Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period     3 years  
Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period     4 years  
Estimated Future Expense in 2018        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated future compensation expense 3.3   $ 3.3  
Estimated Future Expense in 2018 | Restricted Stock and Restricted Stock Units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated future compensation expense 3.3   3.3  
Estimated Future Expense in 2019        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated future compensation expense 10.0   10.0  
Estimated Future Expense in 2019 | Restricted Stock and Restricted Stock Units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated future compensation expense 10.0   10.0  
Estimated Future Expense in 2020        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated future compensation expense 5.3   5.3  
Estimated Future Expense in 2020 | Restricted Stock and Restricted Stock Units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated future compensation expense 5.3   5.3  
Estimated Future Expense in 2021        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated future compensation expense 0.7   0.7  
Estimated Future Expense in 2021 | Restricted Stock and Restricted Stock Units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated future compensation expense $ 0.7   $ 0.7