Summary of Significant Accounting Policies - Summary of Third-Party Payers That Represent More Than 10% of Total Net Revenues and Total Net Accounts Receivable and Their Related Percentage (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Customer Concentration Risk [Member] | Net Revenues [Member] | Medicare and Medicare Advantage [Member]
Concentration Risk [Line Items]
Concentration risk percentage	39.00%	40.00%
Customer Concentration Risk [Member] | Net Revenues [Member] | Blue Cross Blue Shield [Member]
Concentration Risk [Line Items]
Concentration risk percentage	19.00%	11.00%
Customer Concentration Risk [Member] | Net Revenues [Member] | United Healthcare [Member]
Concentration Risk [Line Items]
Concentration risk percentage	12.00%	19.00%
Credit Concentration Risk [Member] | Net Accounts Receivable [Member] | Medicare and Medicare Advantage [Member]
Concentration Risk [Line Items]
Concentration risk percentage	21.00%
Credit Concentration Risk [Member] | Net Accounts Receivable [Member] | Blue Cross Blue Shield [Member]
Concentration Risk [Line Items]
Concentration risk percentage	27.00%
Credit Concentration Risk [Member] | Net Accounts Receivable [Member] | United Healthcare [Member]
Concentration Risk [Line Items]
Concentration risk percentage	15.00%