Significant Accounting Policies - Percentage of Net Service Revenue Earned by Category of Payor (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Significant Accounting Policies [Line Items]
Percentage of net service revenue	100.00%	100.00%	100.00%	100.00%
Medicare [Member]
Significant Accounting Policies [Line Items]
Percentage of net service revenue	80.30%	77.20%	79.80%	77.90%
Medicaid [Member]
Significant Accounting Policies [Line Items]
Percentage of net service revenue	1.30%	1.70%	1.40%	1.90%
Other [Member]
Significant Accounting Policies [Line Items]
Percentage of net service revenue	18.40%	21.10%	18.80%	20.20%