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Benefit Plans (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Benefit Plans      
Maximum contribution to defined contribution retirement plan by employee, as a percentage of compensation 75.00%    
Employer's matching contribution as a percentage of employee's contribution 50.00%    
Expense recognized $ 12.7 $ 11.1 $ 9.6
Maximum      
Benefit Plans      
Maximum employer matching contribution to defined contribution retirement plan, as a percentage of compensation 6.00%