Significant Accounting Policies - Percentage of Net Service Revenue Earned by Type of Ownership or Relationship with Operating Entity (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Significant Accounting Policies [Line Items]
Document Fiscal Year Focus			2018
Health Care Organization, Revenue [Abstract]
Percentage of net service revenue	100.00%	100.00%	100.00%	100.00%
Wholly-owned subsidiaries
Health Care Organization, Revenue [Abstract]
Percentage of net service revenue	59.50%	53.30%	55.50%	53.50%
Equity joint ventures
Health Care Organization, Revenue [Abstract]
Percentage of net service revenue	39.40%	45.10%	43.00%	44.80%
Other
Health Care Organization, Revenue [Abstract]
Percentage of net service revenue	1.10%	1.60%	1.50%	1.70%