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Percentage of Net Revenue and Total Accounts Receivable (Detail)
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Sales Revenue, Net | Payor A [Member]      
Accounts Receivable And Net Revenue [Line Items]      
Concentration Risk, Percentage 17.40% 0.00% [1]  
Sales Revenue, Net | Payor B [Member]      
Accounts Receivable And Net Revenue [Line Items]      
Concentration Risk, Percentage 15.30% 12.00%  
Sales Revenue, Net | Payor C [Member]      
Accounts Receivable And Net Revenue [Line Items]      
Concentration Risk, Percentage 12.00% 10.90%  
Accounts Receivable | Payor D [Member]      
Accounts Receivable And Net Revenue [Line Items]      
Concentration Risk, Percentage 35.30%   34.10%
Accounts Receivable | Payor E [Member]      
Accounts Receivable And Net Revenue [Line Items]      
Concentration Risk, Percentage 42.90%   42.20%
[1] Less than 10% of total net revenues