Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Net operating revenues	$ 1,114,401	$ 1,097,166	$ 1,120,675	$ 1,111,361	$ 1,046,265	$ 1,053,795	$ 1,097,631	$ 1,088,330	$ 4,443,603	$ 4,286,021	$ 3,742,736
Income from operations	76,352	72,098	115,663	91,765	55,745	56,162	101,054	86,886	355,878	299,847	274,790
Net income attributable to Select Medical Holdings Corporation	$ 100,797	$ 18,462	$ 42,055	$ 15,870	$ 20,172	$ 6,471	$ 33,935	$ 54,833	$ 177,184	$ 115,411	$ 130,736
Income per common share:
Basic (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 (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