Earnings per Share - Computation of EPS Under the Two-Class Method (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Net Income Allocation
Net income allocated to common shares - basic	$ 75,360		$ 53,287		$ 143,593	$ 100,766
Participating securities - basic	2,877		1,920		5,449	3,558
Net income allocated to common shares - diluted	75,360		53,287		143,593	100,766
Participating securities - diluted	2,877		1,920		5,449	3,558
Net income attributable to Select Medical Holdings Corporation	$ 78,237	$ 70,805	$ 55,207	$ 49,117	$ 149,042	$ 104,324
Weighted average common shares outstanding, basic (in shares)	122,634		124,897		122,594	126,942
Weighted average common shares outstanding, diluted (in shares)	122,634		124,897		122,594	126,942
Weighted average participating securities outstanding (in shares)	4,681		4,500		4,652	4,482
Basic EPS
Basic EPS (in dollars per share)	$ 0.61		$ 0.43		$ 1.17	$ 0.79
Diluted EPS
Diluted EPS (in dollars per share)	$ 0.61		$ 0.43		$ 1.17	$ 0.79