Earnings per Share - Net Income Attributable to the Company, Common Shares Outstanding, and Participating Securities Outstanding (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Earnings Per Share [Abstract]
Net income	$ 91,860		$ 66,262		$ 177,117	$ 122,188
Less: Net income attributable to non-controlling interests	13,623		11,055		28,075	17,864
Net income attributable to Select Medical Holdings Corporation	78,237	$ 70,805	55,207	$ 49,117	149,042	104,324
Basic EPS
Less: Distributed and undistributed income attributable to participating securities - Basic EPS	2,877		1,920		5,449	3,558
Distributed and undistributed income attributable to common shares	75,360		53,287		143,593	100,766
Diluted EPS
Less: Distributed and undistributed income attributable to participating securities - Diluted EPS	2,877		1,920		5,449	3,558
Distributed and undistributed income attributable to common shares	$ 75,360		$ 53,287		$ 143,593	$ 100,766