Segment Information - Reconciliation of Adjusted EBITDA to Income Before Taxes (Details) - USD ($) $ 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
Segment information
Adjusted EBITDA									$ 537,992	$ 465,807	$ 399,165
Depreciation and amortization									(160,011)	(145,311)	(104,981)
Stock compensation expense									(19,284)	(17,413)	(14,679)
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
Loss on early retirement of debt									(19,719)	(11,626)	0
Equity in earnings of unconsolidated subsidiaries									21,054	19,943	16,811
Non-operating gain (loss)									(49)	42,651	29,647
Interest income (expense)									(154,703)	(170,081)	(112,816)
Income before income taxes									202,461	180,734	208,432
Operating Segments | Long Term Acute Care
Segment information
Adjusted EBITDA									252,679	224,609	258,223
Depreciation and amortization									(45,743)	(43,862)	(45,234)
Stock compensation expense									0	0	0
Acquisition costs									0	0	0
Income from operations									206,936	180,747	212,989
Operating Segments | Inpatient Rehabilitation
Segment information
Adjusted EBITDA									90,041	56,902	69,400
Depreciation and amortization									(20,176)	(12,723)	(8,758)
Stock compensation expense									0	0	0
Acquisition costs									0	0	0
Income from operations									69,865	44,179	60,642
Operating Segments | Outpatient Rehabilitation
Segment information
Adjusted EBITDA									132,533	129,830	98,220
Depreciation and amortization									(24,607)	(22,661)	(13,053)
Stock compensation expense									0	0	0
Acquisition costs									0	0	0
Income from operations									107,926	107,169	85,167
Operating Segments | Concentra
Segment information
Adjusted EBITDA									157,561	143,009	48,301
Depreciation and amortization									(61,945)	(60,717)	(33,644)
Stock compensation expense									(993)	(770)	(1,016)
Acquisition costs									(2,819)	0	(4,715)
Income from operations									91,804	81,522	8,926
Other
Segment information
Adjusted EBITDA									(94,822)	(88,543)	(74,979)
Depreciation and amortization									(7,540)	(5,348)	(4,292)
Stock compensation expense									(18,291)	(16,643)	(13,663)
Acquisition costs									0	(3,236)	0
Income from operations									$ (120,653)	$ (113,770)	$ (92,934)