Significant Accounting Policies - Percentage of Net Service Revenue Earned by Category of Payor (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
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	74.30%	74.00%	74.70%	74.20%
Medicaid revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	1.90%	1.50%	1.80%	1.50%
Other revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	23.80%	24.50%	23.50%	24.30%