Summary of Significant Accounting Policies - Customers Exceeding Ten Percent of Consolidated Net Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Significant customers
Revenue from Contract with Customer, Including Assessed Tax	$ 1,184,960	$ 1,170,117	$ 1,100,075	$ 1,122,379	$ 1,133,775	$ 1,158,543	$ 1,154,289	$ 1,119,006	$ 4,577,531	$ 4,565,613	$ 4,957,522
Discontinued Operations, Held-for-sale
Significant customers
Managed care and other revenue										2,757,511
MCC Business | Discontinued Operations, Held-for-sale
Significant customers
Managed care and other revenue											2,529,386
Maximum
Significant customers
Term of contracts with customers									3 years
Term of contract extensions or renewals									2 years
Virginia Contracts
Significant customers
Revenue from Contract with Customer, Including Assessed Tax									$ 1,013,200	847,800	476,700
New York Contract
Significant customers
Revenue from Contract with Customer, Including Assessed Tax									756,400	836,400	691,600
Massachusetts Contracts
Significant customers
Revenue from Contract with Customer, Including Assessed Tax									$ 709,200	$ 718,900	$ 682,100