Earnings per Share - Computation of EPS Under the Two-Class Method (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Net Income Allocation
Net income allocated to common shares - basic	$ 47,474	$ 106,848
Participating securities - basic	1,643	3,698
Net income allocated to common shares - diluted	47,474	106,848
Participating securities - diluted	1,643	3,698
Net income attributable to Select Medical Holdings Corporation	$ 49,117	$ 110,546
Weighted average common shares outstanding, basic (in shares)	129,010	130,329
Weighted average common shares outstanding, diluted (in shares)	129,010	130,329
Weighted average participating securities outstanding (in shares)	4,464	4,511
Basic EPS
Basic EPS (in dollars per share)	$ 0.37	$ 0.82
Diluted EPS
Diluted EPS (in dollars per share)	$ 0.37	$ 0.82