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, 2020	Mar. 31, 2019
Net Income Allocation, Basic [Abstract]
Net income allocated to common shares	$ 51,307	$ 39,491
Net income allocated to participating securities	1,818	1,343
Net Income Allocation, Diluted [Abstract]
Net income attributable to common shares	51,307	39,491
Net income allocated to participating securities	1,818	1,343
Net income attributable to Select Medical Holdings Corporation	$ 53,125	$ 40,834
Weighted average common shares outstanding, basic (in shares)	129,638	130,821
Weighted average common shares outstanding, diluted (in shares)	129,638	130,861
Weighted average participating securities outstanding (in shares)	4,594	4,449
Basic EPS
Basic EPS (in dollars per share)	$ 0.40	$ 0.30
Diluted EPS
Diluted EPS (in dollars per share)	$ 0.40	$ 0.30