Income per Share (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Contractual dividends									$ 0	$ 0	$ 0
Numerator:
Net income attributable to Select Medical Holdings Corporation	$ 100,797,000	$ 18,462,000	$ 42,055,000	$ 15,870,000	$ 20,172,000	$ 6,471,000	$ 33,935,000	$ 54,833,000	177,184,000	115,411,000	130,736,000
Less: Earnings allocated to unvested restricted stockholders									5,758,000	3,521,000	3,830,000
Net income available to common stockholders									$ 171,426,000	$ 111,890,000	$ 126,906,000
Denominator:
Weighted average shares - basic (in shares)									128,955	127,813	127,478
Effect of dilutive securities:
Stock options (in shares)									171	155	274
Weighted average shares - diluted (in shares)									129,126	127,968	127,752
Income per common share:
Basic income per common share (in dollars per share)	$ 0.75	$ 0.14	$ 0.32	$ 0.12	$ 0.15	$ 0.05	$ 0.26	$ 0.42	$ 1.33	$ 0.88	$ 1.00
Diluted income per common share (in dollars per share)	$ 0.75	$ 0.14	$ 0.32	$ 0.12	$ 0.15	$ 0.05	$ 0.26	$ 0.42	$ 1.33	$ 0.87	$ 0.99