Acquisitions - Additional Information (Detail) (Concentra Inc., USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 22, 2015
Concentra Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Selling price in cash			$ 1,055
Deferred tax asset	53
Revenues	246	998
Pretax earnings	7	22
Assets
Receivables, net	122	115
Property and equipment, net	195	191
Goodwill	480	480
Other intangible assets, net	127	131
Other assets	28	26
Total assets held-for-sale	952	943
Liabilities
Trade accounts payable and accrued expenses	80	90
Other liabilities	113	116
Total liabilities held-for-sale	$ 193	$ 206