Significant Accounting Policies - Percentage of Net Service Revenue Earned by Type of Ownership or Relationship with Operating Entity (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Percentage of net service revenue	100.00%	100.00%	100.00%	100.00%
Equity joint ventures
Disaggregation of Revenue [Line Items]
Percentage of net service revenue	39.50%	47.50%	41.30%	46.10%
Other
Disaggregation of Revenue [Line Items]
Percentage of net service revenue	1.00%	1.90%	1.20%	1.80%
Wholly-owned subsidiaries
Disaggregation of Revenue [Line Items]
Percentage of net service revenue	59.50%	50.60%	57.50%	52.10%