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Common Stock (Employee Stock Purchase Plan) (Details) - USD ($)
$ in Millions
12 Months Ended
May 18, 2012
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation expense   $ 11.2 $ 12.1 $ 13.1
Total fair value of the shares purchased under the plan   0.8 1.0 1.0
Employee Stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Registered shares of common stock (in shares) 500,000      
Percent of common stock fair market value 85.00%      
Offering period 3 months      
Maximum employee compensation payroll deductions may not exceed 10.00%      
Maximum shares employees may purchase in any one offering period (in shares) 1,000      
Share-based compensation expense   $ 0.1 $ 0.1 $ 0.2