Basis of Presentation and Significant Events - COVID-19 (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Adjustments Due to Legislation [Line Items]
Proceeds from Provider Relief Funds				$ 93,300		$ 93,300
Government stimulus income	$ 0	$ 44,400	$ 44,435	0	$ 44,435
Recoupment of fund				65,000
Contract liabilities - deferred revenue	252,936			252,936		$ 317,962
Net service revenue	545,907		$ 487,320	1,070,742	$ 1,000,191
Sequestration Payment Adjustment
Adjustments Due to Legislation [Line Items]
Net service revenue	6,400			12,900
Suspension of LTACH
Adjustments Due to Legislation [Line Items]
Net service revenue	6,900			12,500
Due From Legislation
Adjustments Due to Legislation [Line Items]
Employee-related liabilities	51,800			51,800
Due From Legislation | Other Current Liabilities
Adjustments Due to Legislation [Line Items]
Employee-related liabilities	25,900			25,900
Due From Legislation | Other Noncurrent Liabilities
Adjustments Due to Legislation [Line Items]
Employee-related liabilities	$ 25,900			$ 25,900