Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jun. 01, 2015
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 354	$ 328	$ 309
Capitalized internally developed and purchased software amortization	$ 220	191	$ 172
Concentra Inc.
Property, Plant and Equipment [Line Items]
Property and equipment classified as held-for-sale		$ 191		$ 197