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, 2017	Dec. 31, 2016	Dec. 31, 2015
Health Care organization, Revenue [Abstract]
Percentage of Net Service Revenue	100.00%	100.00%	100.00%
Wholly-owned Subsidiaries [Member]
Health Care organization, Revenue [Abstract]
Percentage of Net Service Revenue	51.00%	57.20%	55.20%
Equity Joint Ventures [Member]
Health Care organization, Revenue [Abstract]
Percentage of Net Service Revenue	47.00%	41.20%	42.90%
Other Arrangements [Member]
Health Care organization, Revenue [Abstract]
Percentage of Net Service Revenue	2.00%	1.60%	1.90%