Debt - Long Term Debt (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total debt	$ 15,498	$ 15,294
Less: unamortized discounts and issuance costs	(1,235)	(1,012)
Total debt, net	14,263	14,282
Less: current portion	(750)	0
Long-term debt, net	$ 13,513	$ 14,282
1.85% due 2018
Debt Instrument [Line Items]
Interest rate	1.85%	1.85%
Long-term debt maturity year	2018	2018
Total debt	$ 750	$ 750
4.25% due 2019
Debt Instrument [Line Items]
Interest rate	4.25%	4.25%
Long-term debt maturity year	2019	2019
Total debt	$ 900	$ 900
2.50% due 2020
Debt Instrument [Line Items]
Interest rate	2.50%	2.50%
Long-term debt maturity year	2020	2020
Total debt	$ 1,250	$ 1,250
3.35% due 2021
Debt Instrument [Line Items]
Interest rate	3.35%	3.35%
Long-term debt maturity year	2021	2021
Total debt	$ 900	$ 900
3.10% due 2023
Debt Instrument [Line Items]
Interest rate	3.10%	3.10%
Long-term debt maturity year	2023	2023
Total debt	$ 500	$ 500
2.90% due 2025
Debt Instrument [Line Items]
Interest rate	2.90%	2.90%
Long-term debt maturity year	2025	2025
Total debt	$ 750	$ 750
3.55% due 2026
Debt Instrument [Line Items]
Interest rate	3.55%	3.55%
Long-term debt maturity year	2026	2026
Total debt	$ 2,000	$ 2,000
3.60% due 2035
Debt Instrument [Line Items]
Interest rate	3.60%	3.60%
Long-term debt maturity year	2035	2035
Total debt	$ 500	$ 500
4.50% and 6.15% due 2036
Debt Instrument [Line Items]
Long-term debt maturity year	2036	2036
Total debt	$ 1,054	$ 1,152
4.50% and 6.15% due 2036 | Minimum
Debt Instrument [Line Items]
Interest rate	4.50%	4.50%
4.50% and 6.15% due 2036 | Maximum
Debt Instrument [Line Items]
Interest rate	6.15%	6.15%
4.85% due 2041
Debt Instrument [Line Items]
Interest rate	4.85%	4.85%
Long-term debt maturity year	2041	2041
Total debt	$ 239	$ 600
4.07% due 2042
Debt Instrument [Line Items]
Interest rate	4.07%	4.07%
Long-term debt maturity year	2042	2042
Total debt	$ 1,336	$ 1,336
3.80% due 2045
Debt Instrument [Line Items]
Interest rate	3.80%	3.80%
Long-term debt maturity year	2045	2045
Total debt	$ 1,000	$ 1,000
4.70% due 2046
Debt Instrument [Line Items]
Interest rate	4.70%	4.70%
Long-term debt maturity year	2046	2046
Total debt	$ 1,326	$ 2,000
4.09% due 2052
Debt Instrument [Line Items]
Interest rate	4.09%	4.09%
Long-term debt maturity year	2052	2052
Total debt	$ 1,578	$ 0
Other notes with rates from 5.50% to 8.50%, due 2023 to 2040
Debt Instrument [Line Items]
Total debt	$ 1,415	$ 1,656
Other notes with rates from 5.50% to 8.50%, due 2023 to 2040 | Minimum
Debt Instrument [Line Items]
Interest rate	5.50%	5.50%
Long-term debt maturity year	2023	2023
Other notes with rates from 5.50% to 8.50%, due 2023 to 2040 | Maximum
Debt Instrument [Line Items]
Interest rate	8.50%	8.50%
Long-term debt maturity year	2040	2040