Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income attributable to Select Medical Holdings Corporation	$ 32,917	$ 18,462	$ 113,167	$ 76,387
Less: Earnings allocated to unvested restricted stockholders	1,098	608	3,732	2,464
Net income available to common stockholders	$ 31,819	$ 17,854	$ 109,435	$ 73,923
Denominator:
Weighted average shares—basic (in shares)	130,387	129,142	129,972	128,745
Effect of dilutive securities:
Stock options (in shares)	60	180	94	171
Weighted average shares—diluted (in shares)	130,447	129,322	130,066	128,916
Income per common share
Basic income per common share: (in dollars per share)	$ 0.24	$ 0.14	$ 0.84	$ 0.57
Diluted income per common share: (in dollars per share)	$ 0.24	$ 0.14	$ 0.84	$ 0.57