Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 173	$ 248	$ 702	$ 902
Effective tax rates	18.60%	36.70%	20.50%	34.10%
Provision for income tax benefit excluding gains on sale	$ 132		$ 377
Provision for tax benefits related to settlement of employee awards	23	$ 4	$ 119	$ 80
Reduction to provision for income taxes for tax credits	28
Effective tax rate			21.00%		35.00%
Liability for unrecognized tax benefits	440		$ 440		$ 439
Unrecognized tax benefits, accrued interest	45		45		44
Unrecognized tax benefits that would impact effective tax rate	$ 145		$ 145		$ 145

Tax cuts and jobs act of 2017 included tax benefits, excluding gains on sales.

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Unrecognized tax benefits including interest accrued net of deferred tax assets.

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

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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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 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 interest expense accrued for an underpayment of income taxes.

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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