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Organization and Description of Business - Credit Risk (Details)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Revenue | Revenue concentration risk          
Concentrations of Credit Risk and Other Risks and Uncertainties          
Concentrations of credit risk (as a percent) 42.00% 40.00% 40.00% 40.00%  
Revenue | Revenue concentration risk | Medicare          
Concentrations of Credit Risk and Other Risks and Uncertainties          
Concentrations of credit risk (as a percent) 29.00% 26.00% 28.00% 26.00%  
Revenue | Revenue concentration risk | UnitedHealthcare          
Concentrations of Credit Risk and Other Risks and Uncertainties          
Concentrations of credit risk (as a percent) 13.00% 14.00% 12.00% 14.00%  
Accounts receivable | Gross receivables concentration risk | Medicare          
Concentrations of Credit Risk and Other Risks and Uncertainties          
Concentrations of credit risk (as a percent)     17.00%   22.00%
Accounts receivable | Gross receivables concentration risk | UnitedHealthcare          
Concentrations of Credit Risk and Other Risks and Uncertainties          
Concentrations of credit risk (as a percent)     11.00%   9.00%