Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Depreciation and fixed asset basis differences, Assets	$ 0	$ 0
Allowances for professional liability and other risks, Assets	345	495
Accounts receivable, Assets	243	351
Compensation, Assets	263	359
Other, Assets	420	794
Deferred tax assets	1,271	1,999
Depreciation and fixed asset basis differences, Liabilities	260	235
Allowances for professional liability and other risks, Liabilities	0	0
Accounts receivable, Liabilities	0	0
Compensation, Liabilities	0	0
Other, Liabilities	501	918
Deferred tax liabilities	$ 761	$ 1,153