Income Taxes - Reconciliation of Expected Income Tax Expense at Federal Statutory Rate of 21 Percent to Company's Applicable Income Tax Expense (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 438	$ 429
State income tax, at statutory rates, net of federal tax benefit	84	89
Tax credits and benefits, net of related expenses	(103)	(115)
Exam resolutions	(49)	(49)
Tax-exempt income	(32)	(32)
Noncontrolling interests	(2)	(1)
Other items	42	41
Applicable income taxes	$ 378	$ 362

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to noncontrolling interest income (loss) exempt from income taxes.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax credits. Including, but not limited to, research credit, foreign tax credit, investment tax credit, and other tax credits.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to income tax settlements. Including, but not limited to, domestic tax settlement, foreign tax settlement, state and local tax settlement, and other tax settlements.

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