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Employee Benefit Plan (Narrative) (Details) - Defined Contribution Plan [Member] - Defined Contribution Benefit Plan [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]      
Employees' vesting percentage in matching contributions (percent) 100.00%    
Contributions made by the Company under the Plan $ 5.5 $ 5.8 $ 6.0
Defined Contribution Plan Tranches One [Member]      
Defined Contribution Plan Disclosure [Line Items]      
Employer matching contribution (percent) 100.00%    
Employee's salary contribution (percent) 3.00%    
Defined Contribution Plan Tranches Two [Member]      
Defined Contribution Plan Disclosure [Line Items]      
Employer matching contribution (percent) 50.00%    
Employee's salary contribution (percent) 2.00%