Summary of Significant Accounting Policies - Percentage of Net Service Revenue Earned by Category of Payor (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	71.00%	74.50%	74.50%
Medicaid [Member]
Significant Accounting Policies [Line Items]
Percentage of net service revenue	1.80%	1.80%	1.50%
Other [Member]
Significant Accounting Policies [Line Items]
Percentage of net service revenue	27.20%	23.70%	24.00%