Earnings per Share - Net Income Attributable to the Company, Common Shares Outstanding, and Participating Securities Outstanding (Details) - USD ($) $ 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
Earnings Per Share [Abstract]
Net income	$ 100,217			$ 104,457			$ 433,639	$ 242,391
Less: Net income attributable to non-controlling interests	23,289			27,511			81,271	60,670
Net income attributable to Select Medical Holdings Corporation	76,928	$ 164,894	$ 110,546	76,946	$ 51,650	$ 53,125	352,368	181,721
Basic EPS
Less: Distributed and undistributed income attributable to participating securities - Basic EPS	2,550			2,666			11,781	6,254
Distributed and undistributed income attributable to common shares	74,378			74,280			340,587	175,467
Diluted EPS
Less: Distributed and undistributed income attributable to participating securities - Diluted EPS	2,550			2,666			11,781	6,254
Distributed and undistributed income attributable to common shares	$ 74,378			$ 74,280			$ 340,587	$ 175,467