Summary of Significant Accounting Policies - Percentage of Net Service Revenue Earned by Type of Ownership or relationship with Operating Entity (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Percentage of net service revenue	100.00%	100.00%	100.00%
Wholly owned subsidiaries
Concentration Risk [Line Items]
Percentage of net service revenue	59.20%	51.10%	57.10%
Equity joint ventures
Concentration Risk [Line Items]
Percentage of net service revenue	40.00%	46.80%	41.20%
Managed or licensed
Concentration Risk [Line Items]
Percentage of net service revenue	0.80%	2.10%	1.70%