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Summary of Significant Accounting Policies - Payor Concentration (Details) - Third-Party Payor Concentration Risk
9 Months Ended 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Medicare | Percentage of Revenues      
Concentration Risk [Line Items]      
Concentration risk percentage 54.00% 47.00%  
Medicare | Percentage of Accounts Receivable (current)      
Concentration Risk [Line Items]      
Concentration risk percentage 11.00%   7.00%
Medicare | Percentage of Accounts Receivable (non-current)      
Concentration Risk [Line Items]      
Concentration risk percentage 0.00%   0.00%
Medicare Advantage plans | Percentage of Revenues      
Concentration Risk [Line Items]      
Concentration risk percentage 28.00% 28.00%  
Medicare Advantage plans | Percentage of Accounts Receivable (current)      
Concentration Risk [Line Items]      
Concentration risk percentage 41.00%   41.00%
Medicare Advantage plans | Percentage of Accounts Receivable (non-current)      
Concentration Risk [Line Items]      
Concentration risk percentage 18.00%   18.00%
United Healthcare | Percentage of Revenues      
Concentration Risk [Line Items]      
Concentration risk percentage 2.00% 7.00%  
United Healthcare | Percentage of Accounts Receivable (current)      
Concentration Risk [Line Items]      
Concentration risk percentage 1.00%   9.00%
United Healthcare | Percentage of Accounts Receivable (non-current)      
Concentration Risk [Line Items]      
Concentration risk percentage 0.00%   0.00%
BlueCross BlueShield plans | Percentage of Revenues      
Concentration Risk [Line Items]      
Concentration risk percentage 8.00% 6.00%  
BlueCross BlueShield plans | Percentage of Accounts Receivable (current)      
Concentration Risk [Line Items]      
Concentration risk percentage 27.00%   25.00%
BlueCross BlueShield plans | Percentage of Accounts Receivable (non-current)      
Concentration Risk [Line Items]      
Concentration risk percentage 47.00%   46.00%