Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Depreciation and fixed asset basis differences, Assets	$ 0	$ 0
Allowances for professional liability and other risks, Assets	355	345
Accounts receivable, Assets	274	243
Compensation, Assets	256	263
Other, Assets	424	420
Deferred tax assets	1,309	1,271
Depreciation and fixed asset basis differences, Liabilities	340	260
Allowances for professional liability and other risks, Liabilities	0	0
Accounts receivable, Liabilities	0	0
Compensation, Liabilities	0	0
Other, Liabilities	491	501
Deferred tax liabilities	$ 831	$ 761