Significant Accounting Policies - Percentage of Net Service Revenue Earned by Category of Payor (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	71.50%	75.30%	71.80%	74.90%
Medicaid revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	2.00%	1.80%	1.80%	1.70%
Managed Care, commercial, and other revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	26.50%	22.90%	26.40%	23.40%