Summary of Significant Accounting Policies - Percentage of Net Service Revenue Earned by Category of Payor (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Significant Accounting Policies [Line Items]
Percentage of net service revenue	100.00%	100.00%	100.00%
Medicare [Member]
Significant Accounting Policies [Line Items]
Percentage of net service revenue	74.50%	75.90%	79.80%
Medicaid [Member]
Significant Accounting Policies [Line Items]
Percentage of net service revenue	1.50%	1.40%	1.40%
Other [Member]
Significant Accounting Policies [Line Items]
Percentage of net service revenue	24.00%	22.70%	18.80%