Percentage of Net Revenue and Total Accounts Receivable (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Sales Revenue, Net | Payor A [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage	14.10%	14.40%
Sales Revenue, Net | Payor B [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage	18.10%	17.80%
Sales Revenue, Net | Payor C [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage	11.10%	12.30%
Sales Revenue, Net | Payor D [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage	11.30%	14.00%
Accounts Receivable | Payor D [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage	23.80%	37.40%
Accounts Receivable | Payor E [Member]
Accounts Receivable And Net Revenue [Line Items]
Concentration Risk, Percentage	30.50%	47.10%