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Retirement Benefit Plan (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended 18 Months Ended
Jun. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Retirement Benefits [Abstract]          
Employer match of employee contributions of first 1% of eligible compensation 100.00%        
Percentage of eligible compensation matched 100% by employer 1.00%        
Employer match of employee contributions of next 5% of eligible compensation 50.00%        
Percentage of eligible compensation matched by employer         100.00%
Employer percentage match of employees contribution         5.00%
Recognized matching contributions from retirement plan (in dollars)   $ 6.5 $ 4.8 $ 3.8