Earnings per Share - Computation of EPS Under the Two-Class Method (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Net Income Allocation
Net income allocated to common shares - basic	$ 74,378			$ 74,280			$ 340,587	$ 175,467
Net income allocated to common shares - diluted	74,378			74,280			340,587	175,467
Net income allocated to participating securities	2,550			2,666			11,781	6,254
Net income attributable to Select Medical Holdings Corporation	$ 76,928	$ 164,894	$ 110,546	$ 76,946	$ 51,650	$ 53,125	$ 352,368	$ 181,721
Weighted average common shares outstanding, basic (in shares)	130,594			129,882			130,441	129,616
Weighted average common shares outstanding, diluted (in shares)	130,594			129,882			130,441	129,616
Weighted average participating securities outstanding (in shares)	4,477			4,662			4,512	4,620
Basic EPS
Basic earnings per share (in dollars per share)	$ 0.57			$ 0.57			$ 2.61	$ 1.35
Diluted EPS
Diluted earnings per share (in dollars per share)	$ 0.57			$ 0.57			$ 2.61	$ 1.35