Significant Accounting Policies - Percentage of Net Service Revenue Earned by Category of Payor (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Significant Accounting Policies [Line Items]
Percentage of net service revenue	100.00%	100.00%	100.00%	100.00%
Medicare revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	71.00%	74.30%	71.50%	74.70%
Medicaid revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	1.80%	1.90%	1.80%	1.80%
Managed Care, commercial, and other revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	27.20%	23.80%	26.70%	23.50%