Significant Accounting Policies - Percentage of Net Service Revenue Earned by Category of Payor (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 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%	75.30%	74.30%	76.90%
Medicaid revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	1.50%	1.40%	1.50%	1.30%
Other revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	24.50%	23.30%	24.20%	21.80%