Significant Accounting Policies - Percentage of Net Service Revenue Earned by Type of Ownership or Relationship with Operating Entity (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
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	50.60%	57.60%	52.10%	57.30%
Equity joint ventures
Health Care Organization, Revenue [Abstract]
Percentage of net service revenue	47.50%	40.70%	46.10%	41.00%
Other
Health Care Organization, Revenue [Abstract]
Percentage of net service revenue	1.90%	1.70%	1.80%	1.70%