Debt - Borrowings and Repayments (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Nov. 30, 2020
Debt
New borrowings				$ 6,428	$ 2,650
Premiums paid on early extinguishment of debt				211	30
Loss on early extinguishment of debt			$ (52)	(220)	(52)
Unamortized discounts and debt issuance costs				81		$ 85
Debt repayments				7,237	$ 5,764
Non-cash financing lease incurred				30
Commercial Paper Program [Member]
Debt
Commercial paper repayments				5,800
Commercial paper borrowings				$ 5,400
Senior Notes, Aggregate [Member]
Debt
Debt instrument face amount	$ 950						$ 2,500
New borrowings	942
Early extinguishment of debt	1,289
Premiums paid on early extinguishment of debt	211	$ 211
Loss on early extinguishment of debt		(220)
Accrued interest	15
Write-off unamortized debt discounts and issuance costs		9
Principal amount outstanding	5,965
Senior Notes, Aggregate [Member] | Cash Flow Hedging [Member]
Debt
Interest expense		$ 6
2.00% senior notes due June 1, 2029 [Member]
Debt
Debt instrument face amount	$ 475
Interest rate (as a percent)	2.00%
2.95% senior notes due June 1, 2041 [Member]
Debt
Debt instrument face amount	$ 475
Interest rate (as a percent)	2.95%
6.125% senior notes due 2039 [Member]
Debt
Interest rate (as a percent)	6.125%
Early extinguishment of debt	$ 6
Principal amount outstanding	$ 252
7.75% senior notes due 2032 [Member]
Debt
Interest rate (as a percent)	7.75%
Early extinguishment of debt	$ 9
Principal amount outstanding	$ 153
7.375% senior notes due 2029 [Member]
Debt
Interest rate (as a percent)	7.375%
Principal amount outstanding	$ 81
4.15% senior notes due 2049 [Member]
Debt
Interest rate (as a percent)	4.15%
Early extinguishment of debt	$ 316
Principal amount outstanding	$ 1,000
4.10% senior notes due 2045 [Member]
Debt
Interest rate (as a percent)	4.10%
Early extinguishment of debt	$ 334
Principal amount outstanding	$ 750
3.90% senior notes due 2035 [Member]
Debt
Interest rate (as a percent)	3.90%
Early extinguishment of debt	$ 153
Principal amount outstanding	$ 450
7.00% senior notes due 2028 [Member]
Debt
Interest rate (as a percent)	7.00%
Early extinguishment of debt	$ 73
Principal amount outstanding	$ 330
7.10% senior notes due 2026 [Member]
Debt
Interest rate (as a percent)	7.10%
Early extinguishment of debt	$ 26
Principal amount outstanding	$ 249
3.50% senior notes due 2024 [Member]
Debt
Interest rate (as a percent)	3.50%
Early extinguishment of debt	$ 194
Principal amount outstanding	$ 350
3.125% senior notes due 2025 [Member]
Debt
Interest rate (as a percent)	3.125%
Early extinguishment of debt	$ 178
Principal amount outstanding	$ 600
3.15% senior notes due 2027 [Member]
Debt
Interest rate (as a percent)	3.15%
Principal amount outstanding	$ 750
2.90% senior notes due 2022 [Member]
Debt
Interest rate (as a percent)	2.90%			2.90%
Principal amount outstanding	$ 500
2.40% senior notes due 2023 [Member]
Debt
Interest rate (as a percent)	2.40%
Principal amount outstanding	$ 500
Tax Exempt Bonds [Member]
Debt
New borrowings				$ 125
Debt repayments				63
Financing leases and other [Member]
Debt
Debt repayments				$ 87
Treasury Lock [Member] | Senior Notes, Aggregate [Member]
Debt
Derivative notional value	450
Derivative payment	$ 8