Significant Accounting Policies - Percentage of Net Service Revenue Earned by Type of Ownership or Relationship with Operating Entity (Detail)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Health Care Organization, Revenue [Abstract]
Percentage of net service revenue	100.00%	100.00%
Wholly-owned subsidiaries
Health Care Organization, Revenue [Abstract]
Percentage of net service revenue	48.70%	54.20%
Equity joint ventures
Health Care Organization, Revenue [Abstract]
Percentage of net service revenue	48.70%	44.10%
Other
Health Care Organization, Revenue [Abstract]
Percentage of net service revenue	2.60%	1.70%