Debt - Summary of Long-Term Debt (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-Term Debt	$ 18,894			$ 18,667
2.133% Notes due June 6, 2019
Debt Instrument [Line Items]
Interest rate	2.133%
Long-Term Debt	$ 0			723
0.368% Notes due June 6, 2019
Debt Instrument [Line Items]
Interest rate	0.368%		0.368%		0.368%
Long-Term Debt	$ 0			823
2.675% Notes due December 15, 2019
Debt Instrument [Line Items]
Interest rate	2.675%
Long-Term Debt	$ 1,123			1,121
2.404% Notes due June 5, 2020
Debt Instrument [Line Items]
Interest rate	2.404%
Long-Term Debt	$ 998			996
3.250% Notes due November 12, 2020
Debt Instrument [Line Items]
Interest rate	3.25%
Long-Term Debt	$ 699			698
Floating Rate Notes due December 29, 2020
Debt Instrument [Line Items]
Long-Term Debt	$ 996			0
3.125% Notes due November 8, 2021
Debt Instrument [Line Items]
Interest rate	3.125%
Long-Term Debt	$ 990			1,003
2.894% Notes due June 6, 2022
Debt Instrument [Line Items]
Interest rate	2.894%
Long-Term Debt	$ 1,793			1,791
Floating Rate Notes due June 6, 2022
Debt Instrument [Line Items]
Long-Term Debt	$ 498			497
1.000% Notes due December 15, 2022
Debt Instrument [Line Items]
Interest rate	1.00%					1.00%
Long-Term Debt	$ 576			586
3.300% Notes due March 1, 2023
Debt Instrument [Line Items]
Interest rate	3.30%
Long-Term Debt	$ 296			296
1.401% Notes due May 24, 2023
Debt Instrument [Line Items]
Interest rate	1.401%	1.401%
Long-Term Debt	$ 346			0
3.875% Notes due May 15, 2024
Debt Instrument [Line Items]
Interest rate	3.875%
Long-Term Debt	$ 182			182
3.363% Notes due June 6, 2024
Debt Instrument [Line Items]
Interest rate	3.363%
Long-Term Debt	$ 1,738			1,736
3.734% Notes due December 15, 2024
Debt Instrument [Line Items]
Interest rate	3.734%
Long-Term Debt	$ 1,368			1,367
3.020% Notes due May 24, 2025
Debt Instrument [Line Items]
Interest rate	3.02%	3.02%
Long-Term Debt	$ 324			0
6.700% Notes due December 1, 2026
Debt Instrument [Line Items]
Long-Term Debt	$ 177			0
1.900% Notes due December 15, 2026
Debt Instrument [Line Items]
Interest rate	1.90%					1.90%
Long-Term Debt	$ 575			585
3.700% Notes due June 6, 2027
Debt Instrument [Line Items]
Interest rate	3.70%
Long-Term Debt	$ 2,383			2,381
7.000% Debentures due August 1, 2027
Debt Instrument [Line Items]
Interest rate	7.00%
Long-Term Debt	$ 156			166
6.700% Debentures due August 1, 2028
Debt Instrument [Line Items]
Interest rate	6.70%
Long-Term Debt	$ 154			164
6.000% Notes due May 15, 2039
Debt Instrument [Line Items]
Interest rate	6.00%
Long-Term Debt	$ 246			246
5.000% Notes due November 12, 2040
Debt Instrument [Line Items]
Interest rate	5.00%
Long-Term Debt	$ 296			296
4.875% Notes due May 15, 2044
Debt Instrument [Line Items]
Interest rate	4.875%
Long-Term Debt	$ 331			331
4.685% Notes due December 15, 2044
Debt Instrument [Line Items]
Interest rate	4.685%
Long-Term Debt	$ 1,159			1,189
4.669% Notes due June 6, 2047
Debt Instrument [Line Items]
Interest rate	4.669%
Long-Term Debt	$ 1,484			1,484
Other long-term debt
Debt Instrument [Line Items]
Long-Term Debt	$ 8			$ 3