Significant Accounting Policies - Percentage of Net Service Revenue Earned by Category of Payor (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
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	75.20%	80.30%	76.30%	79.80%
Medicaid revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	1.70%	1.30%	1.40%	1.40%
Other revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	23.10%	18.40%	22.30%	18.80%