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Common Stock (Employee Stock Purchase Plan) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
May. 18, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation expense $ 14.7 $ 10.5 $ 10.9  
Total fair value of the shares purchased under the plan $ 1.0 1.1 0.9  
Employee Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Registered shares of common stock       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 1,000      
Share-based compensation expense $ 0.2 $ 0.2 $ 0.1