Long-term Debt - Summary of Long-term Debt (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt, carrying amount	$ 15,871	$ 16,463
Commercial paper borrowings
Debt Instrument [Line Items]
Long-term debt, carrying amount	2,393	2,421
4.00% Senior Notes due 2025
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,741	1,740
3.30% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,595	1,594
3.00% Senior Notes due 2020
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,593	1,591
3.65% Senior Notes due 2023
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,492	1,491
2.35% Senior Notes due 2018
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,298	1,297
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	996	996
3.63% Senior Notes due 2022
Debt Instrument [Line Items]
Long-term debt, carrying amount	846	845
0.63% Guaranteed Notes due 2019
Debt Instrument [Line Items]
Long-term debt, carrying amount	704	622
1.50% Guaranteed Notes due 2019
Debt Instrument [Line Items]
Long-term debt, carrying amount	599	536
7.00% Notes due 2038
Debt Instrument [Line Items]
Long-term debt, carrying amount	213	214
4.50% Notes due 2021
Debt Instrument [Line Items]
Long-term debt, carrying amount	136	137
Other
Debt Instrument [Line Items]
Long-term debt, carrying amount	703	1,118
5.95% Notes due 2041
Debt Instrument [Line Items]
Long-term debt, carrying amount	115	116
3.60% Notes due 2022
Debt Instrument [Line Items]
Long-term debt, carrying amount	110	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	83
3.70% Notes due 2024
Debt Instrument [Line Items]
Long-term debt, carrying amount	$ 56	56
6.38% Notes due 2018
Debt Instrument [Line Items]
Long-term debt, carrying amount		$ 297