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Equity Incentive Programs (Equity Incentive Programs and Compensation Expense) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Aggregate shares of common stock reserved (in shares) 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) 1,540,511  
Stock-based compensation $ 5.4 $ 6.0
Estimated future compensation expense $ 25.4  
Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting period 3 years  
Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting period 4 years  
Estimated Future Expense in 2018 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated future compensation expense $ 9.7  
Estimated Future Expense in 2019 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated future compensation expense 9.9  
Estimated Future Expense in 2020 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated future compensation expense 5.1  
Estimated Future Expense in 2021 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated future compensation expense $ 0.7  
Restricted Stock and Restricted Stock Units (RSUs) [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award vesting period 3 years  
Estimated future compensation expense $ 25.4  
Restricted Stock and Restricted Stock Units (RSUs) [Member] | Estimated Future Expense in 2018 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated future compensation expense 9.7  
Restricted Stock and Restricted Stock Units (RSUs) [Member] | Estimated Future Expense in 2019 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated future compensation expense 9.9  
Restricted Stock and Restricted Stock Units (RSUs) [Member] | Estimated Future Expense in 2020 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated future compensation expense 5.1  
Restricted Stock and Restricted Stock Units (RSUs) [Member] | Estimated Future Expense in 2021 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Estimated future compensation expense $ 0.7