Significant Accounting Policies - Percentage of Net Service Revenue Earned by Category of Payor (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
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	74.00%	75.20%	74.20%	76.30%
Medicaid revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	1.50%	1.70%	1.50%	1.40%
Other revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	24.50%	23.10%	24.30%	22.30%