Condensed Consolidating Financial Information - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 160,413	$ 122,549	$ 107,300	$ 99,029
Accounts receivable	746,816	691,732
Prepaid income taxes	19,852	31,387
Other current assets	96,214	75,158
Total Current Assets	1,023,295	920,826
Property and equipment, net	965,390	912,591
Goodwill	3,311,459	2,782,812
Identifiable intangible assets, net	444,847	326,519
Other assets	235,190	184,418
Total Assets	5,980,181	5,127,166	5,123,807
Current Liabilities:
Overdrafts	23,291	29,463
Current portion of long-term debt and notes payable	24,175	22,187
Accounts payable	137,540	128,194
Accrued payroll	176,221	160,562
Accrued vacation	106,026	92,875
Accrued interest	23,648	19,885
Accrued other	188,079	143,166
Income taxes payable	6,129	9,071
Total Current Liabilities	685,109	605,403
Long-term debt, net of current portion	3,305,533	2,677,715
Non-current deferred tax liability	148,227	124,917
Other non-current liabilities	174,170	145,709
Total Liabilities	4,313,039	3,553,744
Redeemable non-controlling interests	779,574	640,818
Stockholders’ Equity:
Common stock	135	134
Capital in excess of par	477,822	463,499
Retained earnings (accumulated deficit)	296,635	359,735
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	774,592	823,368
Non-controlling interests	112,976	109,236
Total Equity	887,568	932,604
Total Liabilities and Equity	5,980,181	5,127,166
Consolidating and Eliminating Adjustments
Current Assets:
Cash and cash equivalents	0	0	0	0
Accounts receivable	0	0
Intercompany receivables	(1,797,866)	(1,658,919)
Prepaid income taxes	0	0
Other current assets	0	0
Total Current Assets	(1,797,866)	(1,658,919)
Property and equipment, net	0	0
Investment in affiliates	(4,572,258)	(4,648,489)
Goodwill	0	0
Identifiable intangible assets, net	0	0
Other assets	(8,968)	(9,989)
Total Assets	(6,379,092)	(6,317,397)
Current Liabilities:
Overdrafts	0	0
Current portion of long-term debt and notes payable	0	0
Accounts payable	0	0
Intercompany payables	(1,797,866)	(1,658,919)
Accrued payroll	0	0
Accrued vacation	0	0
Accrued interest	0	0
Accrued other	0	0
Income taxes payable	0	0
Total Current Liabilities	(1,797,866)	(1,658,919)
Long-term debt, net of current portion	0	0
Non-current deferred tax liability	(8,968)	(9,989)
Other non-current liabilities	0	0
Total Liabilities	(1,806,834)	(1,668,908)
Redeemable non-controlling interests	760,172	624,548
Stockholders’ Equity:
Common stock	0	0
Capital in excess of par	0	0
Retained earnings (accumulated deficit)	(1,485,197)	(1,445,541)
Subsidiary investment	(3,954,581)	(3,932,822)
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	(5,439,778)	(5,378,363)
Non-controlling interests	107,348	105,326
Total Equity	(5,332,430)	(5,273,037)
Total Liabilities and Equity	(6,379,092)	(6,317,397)
Select (Parent Company Only) | Reportable legal entities
Current Assets:
Cash and cash equivalents	77	73	73	11,071
Accounts receivable	0	0
Intercompany receivables	0	0
Prepaid income taxes	16,325	22,704
Other current assets	15,441	13,021
Total Current Assets	31,843	35,798
Property and equipment, net	36,947	39,836
Investment in affiliates	4,448,449	4,524,385
Goodwill	0	0
Identifiable intangible assets, net	3	0
Other assets	38,861	36,494
Total Assets	4,556,103	4,636,513
Current Liabilities:
Overdrafts	23,291	29,463
Current portion of long-term debt and notes payable	15,508	16,635
Accounts payable	12,183	12,504
Intercompany payables	1,722,475	1,598,212
Accrued payroll	12,061	16,736
Accrued vacation	4,394	4,083
Accrued interest	17,775	17,479
Accrued other	60,190	39,219
Income taxes payable	2,764	0
Total Current Liabilities	1,870,641	1,734,331
Long-term debt, net of current portion	1,871,407	2,042,555
Non-current deferred tax liability	0	0
Other non-current liabilities	39,463	36,259
Total Liabilities	3,781,511	3,813,145
Redeemable non-controlling interests	0	0
Stockholders’ Equity:
Common stock	0	0
Capital in excess of par	964,683	947,370
Retained earnings (accumulated deficit)	(190,091)	(124,002)
Subsidiary investment	0	0
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	774,592	823,368
Non-controlling interests	0	0
Total Equity	774,592	823,368
Total Liabilities and Equity	4,556,103	4,636,513
Subsidiary Guarantors | Reportable legal entities
Current Assets:
Cash and cash equivalents	5,627	4,856	6,359	6,467
Accounts receivable	412,454	449,493
Intercompany receivables	1,722,475	1,598,212
Prepaid income taxes	0	5,703
Other current assets	32,281	30,209
Total Current Assets	2,172,837	2,088,473
Property and equipment, net	615,299	623,085
Investment in affiliates	123,809	124,104
Goodwill	2,102,444	2,108,270
Identifiable intangible assets, net	102,423	104,067
Other assets	137,897	98,575
Total Assets	5,254,709	5,146,574
Current Liabilities:
Overdrafts	0	0
Current portion of long-term debt and notes payable	268	740
Accounts payable	76,776	85,489
Intercompany payables	75,391	60,707
Accrued payroll	95,717	98,887
Accrued vacation	58,590	58,355
Accrued interest	16	7
Accrued other	62,049	57,378
Income taxes payable	0	1,302
Total Current Liabilities	368,807	362,865
Long-term debt, net of current portion	72	127
Non-current deferred tax liability	91,487	88,376
Other non-current liabilities	62,354	56,721
Total Liabilities	522,720	508,089
Redeemable non-controlling interests	0	0
Stockholders’ Equity:
Common stock	0	0
Capital in excess of par	0	0
Retained earnings (accumulated deficit)	1,506,874	1,416,857
Subsidiary investment	3,225,115	3,221,628
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	4,731,989	4,638,485
Non-controlling interests	0	0
Total Equity	4,731,989	4,638,485
Total Liabilities and Equity	5,254,709	5,146,574
Non-Guarantor Subsidiaries | Reportable legal entities
Current Assets:
Cash and cash equivalents	3,045	4,561	4,548	5,056
Accounts receivable	123,622	122,728
Intercompany receivables	75,391	60,707
Prepaid income taxes	0	31
Other current assets	14,839	13,031
Total Current Assets	216,897	201,058
Property and equipment, net	90,821	79,013
Investment in affiliates	0	0
Goodwill	0	0
Identifiable intangible assets, net	4,972	5,046
Other assets	33,902	35,440
Total Assets	346,592	320,557
Current Liabilities:
Overdrafts	0	0
Current portion of long-term debt and notes payable	3,631	2,212
Accounts payable	19,948	17,475
Intercompany payables	0	0
Accrued payroll	4,935	4,819
Accrued vacation	13,584	12,295
Accrued interest	27	6
Accrued other	16,637	12,599
Income taxes payable	0	30
Total Current Liabilities	58,762	49,436
Long-term debt, net of current portion	38,787	24,730
Non-current deferred tax liability	882	780
Other non-current liabilities	8,730	8,138
Total Liabilities	107,161	83,084
Redeemable non-controlling interests	0	0
Stockholders’ Equity:
Common stock	0	0
Capital in excess of par	0	0
Retained earnings (accumulated deficit)	(33,181)	(35,942)
Subsidiary investment	272,612	273,415
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	239,431	237,473
Non-controlling interests	0	0
Total Equity	239,431	237,473
Total Liabilities and Equity	346,592	320,557
Non-Guarantor Concentra | Reportable legal entities
Current Assets:
Cash and cash equivalents	151,664	113,059	96,320	76,435
Accounts receivable	210,740	119,511
Intercompany receivables	0	0
Prepaid income taxes	3,527	2,949
Other current assets	33,653	18,897
Total Current Assets	399,584	254,416
Property and equipment, net	222,323	170,657
Investment in affiliates	0	0
Goodwill	1,209,015	674,542
Identifiable intangible assets, net	337,449	217,406
Other assets	33,498	23,898
Total Assets	2,201,869	1,340,919
Current Liabilities:
Overdrafts	0	0
Current portion of long-term debt and notes payable	4,768	2,600
Accounts payable	28,633	12,726
Intercompany payables	0	0
Accrued payroll	63,508	40,120
Accrued vacation	29,458	18,142
Accrued interest	5,830	2,393
Accrued other	49,203	33,970
Income taxes payable	3,365	7,739
Total Current Liabilities	184,765	117,690
Long-term debt, net of current portion	1,395,267	610,303
Non-current deferred tax liability	64,826	45,750
Other non-current liabilities	63,623	44,591
Total Liabilities	1,708,481	818,334
Redeemable non-controlling interests	19,402	16,270
Stockholders’ Equity:
Common stock	0	0
Capital in excess of par	0	0
Retained earnings (accumulated deficit)	11,504	64,626
Subsidiary investment	456,854	437,779
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	468,358	502,405
Non-controlling interests	5,628	3,910
Total Equity	473,986	506,315
Total Liabilities and Equity	2,201,869	1,340,919
Select Medical Corporation
Current Assets:
Cash and cash equivalents	160,413	122,549	$ 107,300	$ 99,029
Accounts receivable	746,816	691,732
Intercompany receivables	0	0
Prepaid income taxes	19,852	31,387
Other current assets	96,214	75,158
Total Current Assets	1,023,295	920,826
Property and equipment, net	965,390	912,591
Investment in affiliates	0	0
Goodwill	3,311,459	2,782,812
Identifiable intangible assets, net	444,847	326,519
Other assets	235,190	184,418
Total Assets	5,980,181	5,127,166
Current Liabilities:
Overdrafts	23,291	29,463
Current portion of long-term debt and notes payable	24,175	22,187
Accounts payable	137,540	128,194
Intercompany payables	0	0
Accrued payroll	176,221	160,562
Accrued vacation	106,026	92,875
Accrued interest	23,648	19,885
Accrued other	188,079	143,166
Income taxes payable	6,129	9,071
Total Current Liabilities	685,109	605,403
Long-term debt, net of current portion	3,305,533	2,677,715
Non-current deferred tax liability	148,227	124,917
Other non-current liabilities	174,170	145,709
Total Liabilities	4,313,039	3,553,744
Redeemable non-controlling interests	779,574	640,818
Stockholders’ Equity:
Common stock	0	0
Capital in excess of par	964,683	947,370
Retained earnings (accumulated deficit)	(190,091)	(124,002)
Subsidiary investment	0	0
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	774,592	823,368
Non-controlling interests	112,976	109,236
Total Equity	887,568	932,604
Total Liabilities and Equity	$ 5,980,181	$ 5,127,166