Acquisitions - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended			1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Concentra	Dec. 31, 2012 Metropolitan Health Networks, Inc.	Jul. 06, 2012 Senior Bridge Family Companies Inc	Dec. 31, 2012 Arcadian Management Services Inc State	Dec. 31, 2012 Anvita, Inc.	Dec. 31, 2012 MCCI Holdings LLC
Business Acquisition [Line Items]
Date of acquisition		Dec. 21, 2010	Dec. 21, 2012	Jul. 06, 2012	Mar. 31, 2012	Dec. 06, 2011	Oct. 29, 2012
Amount per share paid to acquire all outstanding shares			$ 11.25
Estimated transaction value to acquire the entity		$ 805	$ 851
Other intangible assets useful life, years	10 years 4 months 24 days	13 years 8 months 12 days	8 years 4 months 24 days	5 years 2 months 12 days	9 years 8 months 12 days	6 years 6 months
Goodwill acquired		537	569	99	44	116
Amount allocated to other intangible assets		188		14	38	60
Arcadian Member States					15
Accrual of payment of working capital adjustment to sellers		6
Total consideration paid including final working capital determination		811
Excess of consideration over estimated fair value of net tangible assets acquired		725
Acquired goodwill deductible for tax purposes		58
Acquisition-related costs	$ 27