Long-term Debt - Summary of Long-term Debt (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt, carrying amount	$ 14,159	$ 14,875
Commercial paper borrowings
Debt Instrument [Line Items]
Long-term debt, carrying amount	2,600	1,694
4.00% Senior Notes due 2025
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,742	1,741
3.30% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,596	1,595
3.00% Senior Notes due 2020
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,595	1,593
3.65% Senior Notes due 2023
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,493	1,492
4.20% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,100	1,100
2.40% Senior Notes due 2022
Debt Instrument [Line Items]
Long-term debt, carrying amount	997	996
3.63% Senior Notes due 2022
Debt Instrument [Line Items]
Long-term debt, carrying amount	847	846
2.65% Senior Notes due 2022
Debt Instrument [Line Items]
Long-term debt, carrying amount	598	598
2.20% Senior Notes due 2020
Debt Instrument [Line Items]
Long-term debt, carrying amount	498	498
7.00% Notes due 2038
Debt Instrument [Line Items]
Long-term debt, carrying amount	211	212
4.50% Notes due 2021
Debt Instrument [Line Items]
Long-term debt, carrying amount	133	135
5.95% Notes due 2041
Debt Instrument [Line Items]
Long-term debt, carrying amount	115	115
3.60% Notes due 2022
Debt Instrument [Line Items]
Long-term debt, carrying amount	109	110
5.13% Notes due 2043
Debt Instrument [Line Items]
Long-term debt, carrying amount	99	99
4.00% Notes due 2023
Debt Instrument [Line Items]
Long-term debt, carrying amount	82	82
3.70% Notes due 2024
Debt Instrument [Line Items]
Long-term debt, carrying amount	55	56
0.63% Guaranteed Notes due 2019
Debt Instrument [Line Items]
Long-term debt, carrying amount		712
1.50% Guaranteed Notes due 2019
Debt Instrument [Line Items]
Long-term debt, carrying amount		603
Other
Debt Instrument [Line Items]
Long-term debt, carrying amount	$ 289	$ 598