Earnings per Share - Net Income Attributable to the Company, Common Shares Outstanding, and Participating Securities Outstanding (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Earnings Per Share [Abstract]
Net income	$ 55,926	$ 137,214
Less: Net income attributable to non-controlling interests	6,809	26,668
Net income attributable to Select Medical Holdings Corporation	49,117	110,546
Basic EPS
Less: Distributed and undistributed income attributable to participating securities - Basic EPS	1,643	3,698
Distributed and undistributed income attributable to common shares	47,474	106,848
Diluted EPS
Less: Distributed and undistributed income attributable to participating securities - Diluted EPS	1,643	3,698
Distributed and undistributed income attributable to common shares	$ 47,474	$ 106,848