Condensed Consolidated Statements of Changes in Equity and Income (unaudited) - USD ($) $ in Thousands	Total	Total Stockholders’ Equity	Common Stock	Capital in Excess of Par	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Non-controlling Interests
Beginning balance (in shares) at Dec. 31, 2019			134,328,000
Beginning balance at Dec. 31, 2019	$ 929,035	$ 770,972	$ 134	$ 491,038	$ 279,800	$ 0	$ 158,063
Increase (Decrease) in Stockholders' Equity
Net income attributable to Select Medical Holdings Corporation	53,125	53,125			53,125
Net income attributable to non-controlling interests	10,067						10,067
Issuance of restricted stock (in shares)			2,000
Issuance of restricted stock	0		$ 0	0
Forfeitures of unvested restricted stock (in shares)			(15,000)
Forfeitures of unvested restricted stock	0		$ 0	0
Vesting of restricted stock	6,136	6,136		6,136
Repurchase of common shares (in shares)			(492,000)
Repurchase of common shares	(8,691)	(8,691)	$ 0	(5,350)	(3,341)
Issuance of non-controlling interests	1,679						1,679
Distributions to and purchases of non-controlling interests	(6,774)	(2,726)			(2,726)		(4,048)
Redemption value adjustment on non-controlling interests	(10,123)	(10,123)			(10,123)
Other	365	(55)			(55)		420
Ending balance (in shares) at Mar. 31, 2020			133,823,000
Ending balance at Mar. 31, 2020	974,819	808,638	$ 134	491,824	316,680	0	166,181
Beginning balance (in shares) at Dec. 31, 2019			134,328,000
Beginning balance at Dec. 31, 2019	929,035	770,972	$ 134	491,038	279,800	0	158,063
Increase (Decrease) in Stockholders' Equity
Net income attributable to Select Medical Holdings Corporation	181,721
Ending balance (in shares) at Sep. 30, 2020			134,763,000
Ending balance at Sep. 30, 2020	1,218,645	1,038,586	$ 135	500,972	537,479	0	180,059
Beginning balance (in shares) at Mar. 31, 2020			133,823,000
Beginning balance at Mar. 31, 2020	974,819	808,638	$ 134	491,824	316,680	0	166,181
Increase (Decrease) in Stockholders' Equity
Net income attributable to Select Medical Holdings Corporation	51,650	51,650			51,650
Net income attributable to non-controlling interests	12,572						12,572
Issuance of restricted stock (in shares)			200,000
Issuance of restricted stock	0		$ 0	0
Forfeitures of unvested restricted stock (in shares)			(7,000)
Forfeitures of unvested restricted stock	0		$ 0	0
Vesting of restricted stock	6,262	6,262		6,262
Repurchase of common shares (in shares)			(46,000)
Repurchase of common shares	(724)	(724)	$ 0	(441)	(283)
Issuance of non-controlling interests	7	0					7
Distributions to and purchases of non-controlling interests	(483)	(65)		(65)			(418)
Redemption value adjustment on non-controlling interests	127,916	127,916			127,916
Other	411	(794)		(795)	1		1,205
Ending balance (in shares) at Jun. 30, 2020			133,970,000
Ending balance at Jun. 30, 2020	1,172,430	992,883	$ 134	496,785	495,964	0	179,547
Increase (Decrease) in Stockholders' Equity
Net income attributable to Select Medical Holdings Corporation	76,946	76,946			76,946
Net income attributable to non-controlling interests	10,183						10,183
Issuance of restricted stock (in shares)			1,049,000
Issuance of restricted stock	0		$ 1	(1)
Forfeitures of unvested restricted stock (in shares)			(2,000)
Forfeitures of unvested restricted stock	0		$ 0	0
Vesting of restricted stock	6,456	6,456		6,456
Repurchase of common shares (in shares)			(254,000)
Repurchase of common shares	(4,827)	(4,827)	$ 0	(2,366)	(2,461)
Distributions to and purchases of non-controlling interests	(10,338)	(318)		98	(416)		(10,020)
Redemption value adjustment on non-controlling interests	(32,555)	(32,555)			(32,555)
Other	350	1			1		349
Ending balance (in shares) at Sep. 30, 2020			134,763,000
Ending balance at Sep. 30, 2020	$ 1,218,645	1,038,586	$ 135	500,972	537,479	0	180,059
Beginning balance (in shares) at Dec. 31, 2020	134,850,735		134,850,000
Beginning balance at Dec. 31, 2020	$ 1,252,973	1,060,480	$ 135	509,128	553,244	(2,027)	192,493
Increase (Decrease) in Stockholders' Equity
Net income attributable to Select Medical Holdings Corporation	110,546	110,546			110,546
Net income attributable to non-controlling interests	17,042						17,042
Issuance of restricted stock (in shares)			2,000
Issuance of restricted stock	0		$ 0	0
Forfeitures of unvested restricted stock (in shares)			(14,000)
Forfeitures of unvested restricted stock	0		$ 0	0
Vesting of restricted stock	6,173	6,173		6,173
Non-controlling interests acquired in business combination	8,193						8,193
Distributions to and purchases of non-controlling interests	(14,245)	(787)		(787)			(13,458)
Redemption value adjustment on non-controlling interests	(38,405)	(38,405)			(38,405)
Other comprehensive income (loss)	8,151	8,151				8,151
Other	189	(182)		(178)	(4)		371
Ending balance (in shares) at Mar. 31, 2021			134,838,000
Ending balance at Mar. 31, 2021	$ 1,350,617	1,145,976	$ 135	514,336	625,381	6,124	204,641
Beginning balance (in shares) at Dec. 31, 2020	134,850,735		134,850,000
Beginning balance at Dec. 31, 2020	$ 1,252,973	1,060,480	$ 135	509,128	553,244	(2,027)	192,493
Increase (Decrease) in Stockholders' Equity
Net income attributable to Select Medical Holdings Corporation	$ 352,368
Ending balance (in shares) at Sep. 30, 2021	134,144,993		134,144,000
Ending balance at Sep. 30, 2021	$ 1,363,612	1,145,498	$ 134	501,710	639,451	4,203	218,114
Beginning balance (in shares) at Mar. 31, 2021			134,838,000
Beginning balance at Mar. 31, 2021	1,350,617	1,145,976	$ 135	514,336	625,381	6,124	204,641
Increase (Decrease) in Stockholders' Equity
Net income attributable to Select Medical Holdings Corporation	164,894	164,894			164,894
Net income attributable to non-controlling interests	13,241						13,241
Dividends declared for common stockholders ($0.125 per share)	(16,876)	(16,876)			(16,876)
Issuance of restricted stock (in shares)			211,000
Issuance of restricted stock	0		$ 0	0
Forfeitures of unvested restricted stock (in shares)			(2,000)
Forfeitures of unvested restricted stock	0		$ 0	0
Vesting of restricted stock	6,564	6,564		6,564
Repurchase of common shares (in shares)			(42,000)
Repurchase of common shares	(1,610)	(1,610)	$ 0	(707)	(903)
Issuance of non-controlling interests	5,688	(1,051)		(1,051)			6,739
Distributions to and purchases of non-controlling interests	(12,294)	(2,970)		(2,970)			(9,324)
Redemption value adjustment on non-controlling interests	(59,370)	(59,370)			(59,370)
Other comprehensive income (loss)	(1,397)	(1,397)				(1,397)
Other	435	65			65		370
Ending balance (in shares) at Jun. 30, 2021			135,005,000
Ending balance at Jun. 30, 2021	1,449,892	1,234,225	$ 135	516,172	713,191	4,727	215,667
Increase (Decrease) in Stockholders' Equity
Net income attributable to Select Medical Holdings Corporation	76,928	76,928			76,928
Net income attributable to non-controlling interests	10,709						10,709
Dividends declared for common stockholders ($0.125 per share)	(16,940)	(16,940)			(16,940)
Issuance of restricted stock (in shares)			954,000
Issuance of restricted stock	0		$ 1	(1)
Forfeitures of unvested restricted stock (in shares)			(2,000)
Forfeitures of unvested restricted stock	0		$ 0	0
Vesting of restricted stock	7,659	7,659		7,659
Repurchase of common shares (in shares)			(1,813,000)
Repurchase of common shares	(64,440)	(64,440)	$ (2)	(26,712)	(37,726)
Issuance of non-controlling interests	14,238	4,592		4,592			9,646
Distributions to and purchases of non-controlling interests	(18,278)	0					(18,278)
Redemption value adjustment on non-controlling interests	(96,000)	(96,000)			(96,000)
Other comprehensive income (loss)	(524)	(524)				(524)
Other	$ 368	(2)			(2)		370
Ending balance (in shares) at Sep. 30, 2021	134,144,993		134,144,000
Ending balance at Sep. 30, 2021	$ 1,363,612	$ 1,145,498	$ 134	$ 501,710	$ 639,451	$ 4,203	$ 218,114