Percentage of Net Revenue and Total Accounts Receivable (Detail)	3 Months Ended			9 Months Ended			12 Months Ended
	Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017	Dec. 31, 2017
Sales Revenue, Net [Member] | Payor A [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage	15.70%		13.50%	14.00%		13.30%
Sales Revenue, Net [Member] | Payor B [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage	22.60%		14.90%	19.20%		18.60%
Sales Revenue, Net [Member] | Payor C [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage		[1]	10.20%		[1]	11.30%
Sales Revenue, Net [Member] | Payor D [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage	14.80%		10.00%	14.00%		11.50%
Accounts Receivable [Member] | Payor E [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage				19.60%			23.80%
Accounts Receivable [Member] | Payor F [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage				48.80%			30.50%

[1] Less than 10% of total net revenues