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Equity Incentive Programs (Equity Incentive Programs and Compensation Expense) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 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) 1,585,002   1,585,002  
Stock-based compensation $ 3.7 $ 3.7 $ 9.1 $ 9.7
Estimated future compensation expense 22.3   $ 22.3  
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 22.3   $ 22.3  
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 6.5   $ 6.5  
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 6.5   6.5  
Estimated Future Expense in 2019        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated future compensation expense 9.9   9.9  
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 9.9   9.9  
Estimated Future Expense in 2020        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated future compensation expense 5.2   5.2  
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 5.2   5.2  
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