Significant Accounting Policies - Percentage of Net Service Revenue Earned by Category of Payor (Detail)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Significant Accounting Policies [Line Items]
Percentage of net service revenue	100.00%	100.00%
Medicare revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	72.10%	74.40%
Medicaid revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	1.60%	1.70%
Other revenue
Significant Accounting Policies [Line Items]
Percentage of net service revenue	26.30%	23.90%