Long-term Debt - Summary of Long-term Debt (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt, carrying amount	$ 16,449	$ 14,644
4.00% Senior Notes due 2025
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,743	1,742
3.30% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,597	1,596
3.00% Senior Notes due 2020
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,596	1,596
3.65% Senior Notes due 2023
Debt Instrument [Line Items]
Long-term debt, carrying amount	1,494	1,493
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	997
3.63% Senior Notes due 2022
Debt Instrument [Line Items]
Long-term debt, carrying amount	847	847
3.75% Senior Notes due 2024
Debt Instrument [Line Items]
Long-term debt, carrying amount	745
4.30% Senior Notes due 2029
Debt Instrument [Line Items]
Long-term debt, carrying amount	844
1.00% Guaranteed Notes due 2026
Debt Instrument [Line Items]
Long-term debt, carrying amount	671	678
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	499	499
7.00% Notes due 2038
Debt Instrument [Line Items]
Long-term debt, carrying amount	209	210
4.50% Notes due 2021
Debt Instrument [Line Items]
Long-term debt, carrying amount	132	132
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	109
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	81	82
3.70% Notes due 2024
Debt Instrument [Line Items]
Long-term debt, carrying amount	55	55
Other
Debt Instrument [Line Items]
Long-term debt, carrying amount	263	263
Commercial paper borrowings
Debt Instrument [Line Items]
Long-term debt, carrying amount	$ 2,655	$ 2,433