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Accounting Policies - Schedule of Revenue Deductions Related to Uninsured Accounts (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Regulatory Assets [Abstract]      
Charity care $ 4,861 $ 4,151 $ 3,682
Uninsured discounts 14,520 13,047 10,692
Provision for doubtful accounts 4,039 3,257 3,913
Total uncompensated care $ 23,420 $ 20,455 $ 18,287
Charity care, ratio 21.00% 20.00% 20.00%
Uninsured discounts, ratio 62.00% 64.00% 59.00%
Provision for doubtful accounts, ratio 17.00% 16.00% 21.00%
Total uncompensated care, ratio 100.00% 100.00% 100.00%