Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Components Of Deferred Tax Assets And Liabilities [Line Items]
Depreciation and fixed asset basis differences, Liabilities	$ 292	$ 402
Other, Liabilities	595	511
Deferred tax liabilities	887	913
Allowances for professional liability and other risks, Assets	355	337
Accounts receivable, Assets	405	706
Compensation, Assets	237	216
Other, Assets	744	698
Deferred tax assets	$ 1,741	$ 1,957