Condensed Consolidating Financial Information - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 141,029	$ 122,549	$ 73,799	$ 99,029
Accounts receivable	775,610	691,732
Prepaid income taxes	14,488	31,387
Other current assets	88,215	75,158
Total Current Assets	1,019,342	920,826
Property and equipment, net	965,844	912,591
Goodwill	3,314,606	2,782,812
Identifiable intangible assets, net	451,932	326,519
Other assets	213,076	184,418
Total Assets	5,964,800	5,127,166	5,054,211
Current Liabilities:
Overdrafts	23,292	29,463
Current portion of long-term debt and notes payable	24,479	22,187
Accounts payable	131,830	128,194
Accrued payroll	149,967	160,562
Accrued vacation	109,958	92,875
Accrued interest	13,293	19,885
Accrued other	170,067	143,166
Income taxes payable	4,425	9,071
Total Current Liabilities	627,311	605,403
Long-term debt, net of current portion	3,386,209	2,677,715
Non-current deferred tax liability	150,694	124,917
Other non-current liabilities	172,427	145,709
Total Liabilities	4,336,641	3,553,744
Redeemable non-controlling interests	616,232	640,818
Stockholders’ Equity:
Common stock	134	134
Capital in excess of par	474,812	463,499
Retained earnings (accumulated deficit)	420,525	359,735
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	895,471	823,368
Non-controlling interests	116,456	109,236
Total Equity	1,011,927	932,604
Total Liabilities and Equity	5,964,800	5,127,166
Consolidating and Eliminating Adjustments
Current Assets:
Cash and cash equivalents	0	0	0	0
Accounts receivable	0	0
Intercompany receivables	(1,753,169)	(1,658,919)
Prepaid income taxes	0	0
Other current assets	0	0
Total Current Assets	(1,753,169)	(1,658,919)
Property and equipment, net	0	0
Investment in affiliates	(4,699,146)	(4,648,489)
Goodwill	0	0
Identifiable intangible assets, net	0	0
Other assets	(9,598)	(9,989)
Total Assets	(6,461,913)	(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	(1,753,169)	(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	(1,753,169)	(1,658,919)
Long-term debt, net of current portion	0	0
Non-current deferred tax liability	(9,598)	(9,989)
Other non-current liabilities	0	0
Total Liabilities	(1,762,767)	(1,668,908)
Redeemable non-controlling interests	597,683	624,548
Stockholders’ Equity:
Common stock	0	0
Capital in excess of par	0	0
Retained earnings (accumulated deficit)	(1,454,279)	(1,445,541)
Subsidiary investment	(3,953,576)	(3,932,822)
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	(5,407,855)	(5,378,363)
Non-controlling interests	111,026	105,326
Total Equity	(5,296,829)	(5,273,037)
Total Liabilities and Equity	(6,461,913)	(6,317,397)
Select (Parent Company Only) | Reportable legal entities
Current Assets:
Cash and cash equivalents	15,074	73	72	11,071
Accounts receivable	0	0
Intercompany receivables	0	0
Prepaid income taxes	10,691	22,704
Other current assets	13,897	13,021
Total Current Assets	39,662	35,798
Property and equipment, net	37,157	39,836
Investment in affiliates	4,566,506	4,524,385
Goodwill	0	0
Identifiable intangible assets, net	3	0
Other assets	35,011	36,494
Total Assets	4,678,339	4,636,513
Current Liabilities:
Overdrafts	23,292	29,463
Current portion of long-term debt and notes payable	17,390	16,635
Accounts payable	9,929	12,504
Intercompany payables	1,672,980	1,598,212
Accrued payroll	8,649	16,736
Accrued vacation	4,551	4,083
Accrued interest	6,926	17,479
Accrued other	39,928	39,219
Income taxes payable	2,387	0
Total Current Liabilities	1,786,032	1,734,331
Long-term debt, net of current portion	1,958,529	2,042,555
Non-current deferred tax liability	0	0
Other non-current liabilities	38,307	36,259
Total Liabilities	3,782,868	3,813,145
Redeemable non-controlling interests	0	0
Stockholders’ Equity:
Common stock	0	0
Capital in excess of par	959,173	947,370
Retained earnings (accumulated deficit)	(63,702)	(124,002)
Subsidiary investment	0	0
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	895,471	823,368
Non-controlling interests	0	0
Total Equity	895,471	823,368
Total Liabilities and Equity	4,678,339	4,636,513
Subsidiary Guarantors | Reportable legal entities
Current Assets:
Cash and cash equivalents	6,360	4,856	5,533	6,467
Accounts receivable	439,614	449,493
Intercompany receivables	1,672,980	1,598,212
Prepaid income taxes	0	5,703
Other current assets	28,254	30,209
Total Current Assets	2,147,208	2,088,473
Property and equipment, net	616,853	623,085
Investment in affiliates	132,640	124,104
Goodwill	2,105,717	2,108,270
Identifiable intangible assets, net	103,119	104,067
Other assets	119,499	98,575
Total Assets	5,225,036	5,146,574
Current Liabilities:
Overdrafts	0	0
Current portion of long-term debt and notes payable	527	740
Accounts payable	71,355	85,489
Intercompany payables	80,189	60,707
Accrued payroll	89,842	98,887
Accrued vacation	61,895	58,355
Accrued interest	11	7
Accrued other	64,563	57,378
Income taxes payable	0	1,302
Total Current Liabilities	368,382	362,865
Long-term debt, net of current portion	90	127
Non-current deferred tax liability	89,230	88,376
Other non-current liabilities	63,983	56,721
Total Liabilities	521,685	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)	1,478,075	1,416,857
Subsidiary investment	3,225,276	3,221,628
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	4,703,351	4,638,485
Non-controlling interests	0	0
Total Equity	4,703,351	4,638,485
Total Liabilities and Equity	5,225,036	5,146,574
Non-Guarantor Subsidiaries | Reportable legal entities
Current Assets:
Cash and cash equivalents	4,396	4,561	4,946	5,056
Accounts receivable	129,585	122,728
Intercompany receivables	80,189	60,707
Prepaid income taxes	0	31
Other current assets	10,075	13,031
Total Current Assets	224,245	201,058
Property and equipment, net	84,834	79,013
Investment in affiliates	0	0
Goodwill	0	0
Identifiable intangible assets, net	4,968	5,046
Other assets	34,360	35,440
Total Assets	348,407	320,557
Current Liabilities:
Overdrafts	0	0
Current portion of long-term debt and notes payable	967	2,212
Accounts payable	18,508	17,475
Intercompany payables	0	0
Accrued payroll	3,650	4,819
Accrued vacation	14,091	12,295
Accrued interest	20	6
Accrued other	15,172	12,599
Income taxes payable	0	30
Total Current Liabilities	52,408	49,436
Long-term debt, net of current portion	33,417	24,730
Non-current deferred tax liability	820	780
Other non-current liabilities	9,482	8,138
Total Liabilities	96,127	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)	(20,267)	(35,942)
Subsidiary investment	272,547	273,415
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	252,280	237,473
Non-controlling interests	0	0
Total Equity	252,280	237,473
Total Liabilities and Equity	348,407	320,557
Non-Guarantor Concentra | Reportable legal entities
Current Assets:
Cash and cash equivalents	115,199	113,059	63,248	76,435
Accounts receivable	206,411	119,511
Intercompany receivables	0	0
Prepaid income taxes	3,797	2,949
Other current assets	35,989	18,897
Total Current Assets	361,396	254,416
Property and equipment, net	227,000	170,657
Investment in affiliates	0	0
Goodwill	1,208,889	674,542
Identifiable intangible assets, net	343,842	217,406
Other assets	33,804	23,898
Total Assets	2,174,931	1,340,919
Current Liabilities:
Overdrafts	0	0
Current portion of long-term debt and notes payable	5,595	2,600
Accounts payable	32,038	12,726
Intercompany payables	0	0
Accrued payroll	47,826	40,120
Accrued vacation	29,421	18,142
Accrued interest	6,336	2,393
Accrued other	50,404	33,970
Income taxes payable	2,038	7,739
Total Current Liabilities	173,658	117,690
Long-term debt, net of current portion	1,394,173	610,303
Non-current deferred tax liability	70,242	45,750
Other non-current liabilities	60,655	44,591
Total Liabilities	1,698,728	818,334
Redeemable non-controlling interests	18,549	16,270
Stockholders’ Equity:
Common stock	0	0
Capital in excess of par	0	0
Retained earnings (accumulated deficit)	(3,529)	64,626
Subsidiary investment	455,753	437,779
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	452,224	502,405
Non-controlling interests	5,430	3,910
Total Equity	457,654	506,315
Total Liabilities and Equity	2,174,931	1,340,919
Select Medical Corporation
Current Assets:
Cash and cash equivalents	141,029	122,549	$ 73,799	$ 99,029
Accounts receivable	775,610	691,732
Intercompany receivables	0	0
Prepaid income taxes	14,488	31,387
Other current assets	88,215	75,158
Total Current Assets	1,019,342	920,826
Property and equipment, net	965,844	912,591
Investment in affiliates	0	0
Goodwill	3,314,606	2,782,812
Identifiable intangible assets, net	451,932	326,519
Other assets	213,076	184,418
Total Assets	5,964,800	5,127,166
Current Liabilities:
Overdrafts	23,292	29,463
Current portion of long-term debt and notes payable	24,479	22,187
Accounts payable	131,830	128,194
Intercompany payables	0	0
Accrued payroll	149,967	160,562
Accrued vacation	109,958	92,875
Accrued interest	13,293	19,885
Accrued other	170,067	143,166
Income taxes payable	4,425	9,071
Total Current Liabilities	627,311	605,403
Long-term debt, net of current portion	3,386,209	2,677,715
Non-current deferred tax liability	150,694	124,917
Other non-current liabilities	172,427	145,709
Total Liabilities	4,336,641	3,553,744
Redeemable non-controlling interests	616,232	640,818
Stockholders’ Equity:
Common stock	0	0
Capital in excess of par	959,173	947,370
Retained earnings (accumulated deficit)	(63,702)	(124,002)
Subsidiary investment	0	0
Total Select Medical Holdings Corporation and Select Medical Corporation Stockholders’ Equity	895,471	823,368
Non-controlling interests	116,456	109,236
Total Equity	1,011,927	932,604
Total Liabilities and Equity	$ 5,964,800	$ 5,127,166