Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Depreciation and fixed asset basis differences, Assets	$ 0	$ 0
Allowances for professional liability and other risks, Assets	407	376
Accounts receivable, Assets	283	307
Compensation, Assets	487	292
Right-of-use lease obligations	416	369
Other, Assets	485	461
Deferred tax assets	2,078	1,805
Depreciation and fixed asset basis differences, Liabilities	678	601
Allowances for professional liability and other risks, Liabilities	0	0
Accounts receivable, Liabilities	0	0
Compensation, Liabilities	0	0
Right-of-use lease assets and obligations	409	366
Other, Liabilities	606	538
Deferred tax liabilities	$ 1,693	$ 1,505