The Company, Business Activities and Basis of Presentation - Summary of Third-Party Payers That Represent More Than 10% of Total Net Revenues and Total Net Accounts Receivable and Their Related Percentage (Detail)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Customer Concentration Risk [Member] | Net Revenues [Member] | Blue Cross Blue Shield [Member]
Concentration Risk [Line Items]
Concentration risk percentage	14.00%	22.00%
Customer Concentration Risk [Member] | Net Revenues [Member] | Medicare and Medicare Advantage [Member]
Concentration Risk [Line Items]
Concentration risk percentage	45.00%	39.00%
Customer Concentration Risk [Member] | Net Revenues [Member] | United Healthcare [Member]
Concentration Risk [Line Items]
Concentration risk percentage	12.00%	22.00%
Credit Concentration Risk [Member] | Net Accounts Receivable [Member] | Blue Cross Blue Shield [Member]
Concentration Risk [Line Items]
Concentration risk percentage	22.00%	22.00%
Credit Concentration Risk [Member] | Net Accounts Receivable [Member] | Medicare and Medicare Advantage [Member]
Concentration Risk [Line Items]
Concentration risk percentage	19.00%	17.00%
Credit Concentration Risk [Member] | Net Accounts Receivable [Member] | United Healthcare [Member]
Concentration Risk [Line Items]
Concentration risk percentage	16.00%	15.00%