Earnings per Share - Computation of EPS Under the Two-Class Method (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Net Income Allocation
Net income allocated to common shares - basic	$ 234,718	$ 153,385	$ 388,790
Net income allocated to participating securities - basic	8,773	5,609	13,435
Net income allocated to common shares - diluted	234,718	153,385	388,790
Net income allocated to participating securities - diluted	8,773	5,609	13,435
Net income attributable to Select Medical Holdings Corporation	$ 243,491	$ 158,994	$ 402,225
Weighted average common shares outstanding, basic (in shares)	123,105	124,628	130,249
Weighted average common shares outstanding, diluted (in shares)	123,105	124,628	130,249
Weighted average participating securities outstanding (in shares)	4,601	4,557	4,501
Basic EPS
Basic EPS (in dollars per share)	$ 1.91	$ 1.23	$ 2.98
Diluted EPS
Diluted EPS (in dollars per share)	$ 1.91	$ 1.23	$ 2.98