Income Taxes - Reconciliation of Expected Income Tax Expense at Federal Statutory Rate of 21 Percent and 35 Percent to Company's Applicable Income Tax Expense (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Tax at statutory rate	$ 1,822	$ 2,631	$ 2,837
State income tax, at statutory rates, net of federal tax benefit	352	281	244
Revaluation of tax related assets and liabilities		(910)
Tax credits and benefits, net of related expenses	(513)	(774)	(710)
Tax-exempt income	(130)	(200)	(196)
Noncontrolling interests	(6)	(12)	(20)
Nondeductible legal and regulatory expenses	52	213	30
Other items	(23)	35	(24)
Applicable income taxes	$ 1,554	$ 1,264	$ 2,161

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to 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 increase (decrease) in the income tax rates.

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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 (loss) exempt from income taxes.

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