Debt - Assets and Liabilities Related to Asset-backed Debt Arrangements (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Account receivable, net
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets	$ 9,158	$ 8,101
Prepaid expenses and other
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets	858	636
Other assets
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets	3,147	2,680
Accounts payable and accrued liabilities
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Liabilities	6	5
Short-term portion of long-term debt
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Liabilities	4,436	1,932
Long-term debt
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Liabilities	$ 4,763	$ 6,955