Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018
Business Acquisition [Line Items]
Effective income tax rate reconciliation, percent	18.00%	22.00%
Income tax provision	$ 653	$ 717
Federal statutory income tax rate, percent				24.50%
Tax cuts and jobs act of 2017, incomplete accounting, deferred tax liability, provisional income tax (expense) benefit		1,100
Tax cuts and jobs act of 2017, incomplete accounting, transition tax accumulated foreign earnings, provisional income tax expense (benefit)		$ 1,100
Increase in unrecognized tax benefits	$ 38
Forecast
Business Acquisition [Line Items]
Federal statutory income tax rate, percent			21.00%