Fair Value of Financial Instruments - Schedule of Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 3,446,530	$ 3,473,582
Senior notes | Select Medical Corporation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate of debt (as a percent)	6.25%	6.25%
Carrying Value	$ 1,239,470	$ 1,241,820
Fair Value	1,285,148	1,316,875
Senior notes | Select Medical Corporation | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	1,239,470	1,241,820
Fair Value	1,285,148	1,316,875
Term loans | Select Medical Corporation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	2,089,032	2,085,895
Fair Value	2,087,661	2,082,403
Term loans | Select Medical Corporation | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	2,089,032	2,085,895
Fair Value	$ 2,087,661	$ 2,082,403