Acquisitions - Concentra Acquisition (Details) - USD ($) $ in Thousands		12 Months Ended
	Jun. 01, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consideration given for identifiable net assets and goodwill acquired to the net cash paid
Goodwill		$ 2,782,812	$ 2,751,000	$ 2,314,624
Net cash paid		27,390	472,206	1,061,628
Concentra Inc
Consideration given for identifiable net assets and goodwill acquired to the net cash paid
Contributed net revenue		1,000,000	1,000,000	585,200
Contributed net income		$ 68,700	$ 19,700	(10,000)
Concentra Inc | Humana | MJ Acquisition Corporation
Acquisitions
Voting equity interests acquired	100.00%
Acquisition costs				$ 4,700
Consideration given for identifiable net assets and goodwill acquired to the net cash paid
Cash and cash equivalents	$ 3,772
Identifiable tangible assets, excluding cash and cash equivalents	406,926
Identifiable intangible assets	254,990
Goodwill	651,152
Total assets	1,316,840
Total liabilities	248,797
Acquired non-controlling interests	17,084
Net assets acquired	1,050,959
Net cash paid	1,047,187
Estimated value of goodwill, deductible for tax purposes	$ 23,900