Debt (Narrative) (Details)			3 Months Ended	12 Months Ended
	May 01, 2018 shares	Oct. 02, 2012 $ / shares	Mar. 31, 2018 USD ($)	Dec. 31, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Nov. 23, 2020 USD ($)	Aug. 17, 2020 USD ($)	May 05, 2020 USD ($)	Sep. 09, 2019 USD ($)	Aug. 15, 2019 USD ($)	Jul. 16, 2018 USD ($)	Mar. 02, 2018 USD ($)	Jan. 15, 2018 USD ($)	May 12, 2015 USD ($) shares	Oct. 09, 2012 USD ($)
Debt Instrument [Line Items]
Issuance of common stock under Equity Units stock purchase contracts, shares | shares	6,000,000.0
Long-term debt face amount				$ 159,000,000
Gain (loss) on extinguishment of debt				(36,000,000)	$ (2,000,000)	$ (11,000,000)
Long-term debt				20,035,000,000	19,385,000,000
Interest paid				794,000,000	755,000,000	728,000,000
Future maturities of debt in 2021				950,000,000
Future maturities of debt in 2022				1,597,000,000
Future maturities of debt in 2023				1,027,000,000
Future maturities of debt in 2024				1,643,000,000
Future maturities of debt in 2025				1,253,000,000
Future maturities of debt Thereafter				13,565,000,000
Short-term borrowings				0	700,000,000
Repayments of Long-term Debt				$ 1,932,000,000	1,123,000,000	1,684,000,000
Debt Instrument, Convertible, Conversion Price | $ / shares				$ 71.0187
Debt Instrument, Convertible, Conversion Ratio, Shares				14.0808
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Covenant Compliance				As of December 31, 2020, our debt-to-capital ratio, as defined and calculated under the credit facilities, was 37.6%. We do not believe the restrictions contained in any of our credit facility covenants materially affect our financial or operating flexibility. As of December 31, 2020, we were in compliance with all of the debt covenants under these credit facilities.
Debt Instrument, Covenant Description				Our ability to borrow under these credit facilities is subject to compliance with certain covenants, including covenants requiring us to maintain a defined debt-to-capital ratio of not more than 60%, subject to increase in certain circumstances set forth in the applicable credit agreement.
Senior Convertible Debentures [Member]
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt				$ (6,000,000)	(2,000,000)	7,000,000
Repurchased Face Amount				56,000,000	15,000,000
Repayments of Long-term Debt				222,000,000	273,000,000	402,000,000
Aggregate principal amount of convertible debentures surrendered for conversion					57,000,000	109,000,000
Equity Unit Purchase Agreements [Member]
Debt Instrument [Line Items]
Equity Units Stock Purchase Contracts Settlement Rate | shares	0.2412
Issuance of common stock under Equity Units stock purchase contracts, shares | shares	6,000,000
Equity Units Issued | shares															25,000,000
Long-term debt face amount															$ 1,250,000,000
Equity Unit Stated Value Per Unit															$ 50
Equity Units Ownership Interest Percentage In Remarketable Subordinated Notes															5.00%
Remarketable Subordinated Notes [Member]
Debt Instrument [Line Items]
Early repayment of senior debt			$ 4,000,000
Remarketable Subordinated Notes [Member] | 1.900%, due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt face amount															$ 1,000,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate															1.90%
Remarketable Subordinated Notes [Member] | 4.101%, due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt face amount													$ 1,250,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate													4.101%
Remarketable Subordinated Notes [Member] | 4.550%, due 2048 [Member]
Debt Instrument [Line Items]
Long-term debt face amount													$ 850,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate													4.55%
Remarketable Subordinated Notes [Member] | Remarketable Subordinated Notes [Member]
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt			$ (18,000,000)
Commercial Paper [Member]
Debt Instrument [Line Items]
Commercial paper authorized				3,500,000,000
Commercial paper				$ 250,000,000	$ 400,000,000
Weighted average interest rate on commercial paper borrowings				0.16%	1.8528%
Senior Convertible Debentures [Member] | 2.750%, due 2042 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate				2.75%
Long-term debt face amount				$ 1,000												$ 1,500,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate				2.75%	2.75%											2.75%
Maturity date on long-term debt				Oct. 15, 2042
Long-term debt				$ 108,000,000	$ 145,000,000
Interest expense				$ 6,000,000	9,000,000	12,000,000
Remaining amortization period debt discount				22 years
Effective interest rate for unamortized debt discount				5.13%
Interest expense resulting from the stated interest rate				$ 5,000,000	7,000,000	10,000,000
Interest expense resulting from amortization of the debt discount				$ 1,000,000	$ 2,000,000	$ 2,000,000
Number of trading days in 30 day period greater than 130%				20
Number of trading days exceeding 130%				30 days
Product of Conversion to Stock Price				130.00%
Required days trading price less than 98%				5 days
Measurement Period for testing				10 days
Percent of Conversion Price to Closing Price				98.00%
Earliest Date for Conversion				Apr. 15, 2042
Date debentures redeemable at our option				Oct. 20, 2022
Conversion premium (%)		25.00%
Closing Price Company Stock | $ / shares		$ 60.46		$ 321.09
If-converted value per dollar above the conversion price				$ 560,000,000
Debt Instrument, Convertible, Conversion Price | $ / shares		$ 75.575
Debt Instrument, Convertible, Conversion Ratio, Shares		13.2319
Senior Unsecured Notes [Member] | 2.300%, due 2018 [Member]
Debt Instrument [Line Items]
Repurchased face amount												$ 650,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate												2.30%
Senior Unsecured Notes [Member] | 1.875%, due 2018 [Member]
Debt Instrument [Line Items]
Repurchased face amount														$ 625,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate														1.875%
Senior Unsecured Notes [Member] | 2.250% due 2019 [Member]
Debt Instrument [Line Items]
Repurchased face amount											$ 850,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate											2.25%
Senior Unsecured Notes [Member] | 2.500% due 2020 [Member]
Debt Instrument [Line Items]
Repurchased face amount							$ 900,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate				2.50%	2.50%		2.50%
Long-term debt				$ 0	$ 899,000,000
Senior Unsecured Notes [Member] | 2.950% due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate				2.95%	2.95%
Long-term debt				$ 749,000,000	$ 747,000,000
Senior Unsecured Notes [Member] | 3.350% due 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate				3.35%	3.35%
Long-term debt				$ 847,000,000	$ 846,000,000
Senior Unsecured Notes [Member] | 2.375%, due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt face amount									$ 400,000,000	$ 850,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate									2.375%	2.375%
Senior Unsecured Notes [Member] | 3.650% due 2027 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate				3.65%	3.65%
Long-term debt				$ 1,591,000,000	$ 1,590,000,000
Senior Unsecured Notes [Member] | 4.101%, due 2028 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate				4.101%	4.101%
Long-term debt				$ 1,257,000,000	$ 1,253,000,000
Senior Unsecured Notes [Member] | 2.875%, due 2029 [Member]
Debt Instrument [Line Items]
Long-term debt face amount										$ 825,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate										2.875%
Senior Unsecured Notes [Member] | 4.375% due 2047 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate				4.375%	4.375%
Long-term debt				$ 1,387,000,000	$ 1,386,000,000
Senior Unsecured Notes [Member] | 4.550%, due 2048 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate				4.55%	4.55%
Long-term debt				$ 839,000,000	$ 838,000,000
Senior Unsecured Notes [Member] | 3.700%, due 2049 [Member]
Debt Instrument [Line Items]
Long-term debt face amount										$ 825,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate										3.70%
Senior Unsecured Notes [Member] | 4.350%, due 2020 [Member]
Debt Instrument [Line Items]
Repurchased face amount								$ 700,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate				4.35%	4.35%			4.35%
Long-term debt				$ 0	$ 699,000,000
Senior Unsecured Notes [Member] | 2.250%, due 2030 [Member]
Debt Instrument [Line Items]
Long-term debt face amount									$ 1,100,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate				2.25%					2.25%
Long-term debt				$ 1,089,000,000	0
Senior Unsecured Notes [Member] | 3.125% due 2050 [Member]
Debt Instrument [Line Items]
Long-term debt face amount									$ 1,000,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate				3.125%					3.125%
Long-term debt				$ 987,000,000	0
Other senior unsecured notes [Member]
Debt Instrument [Line Items]
Gain (loss) on extinguishment of debt				(30,000,000)
Repurchased Face Amount				79,000,000
Extinguishment of Debt, Amount				109,000,000
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Short-term FHLB borrowings outstanding				0	$ 650,000,000
Debt instrument interest rate					1.664%
Subsidiary Credit Facilities
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity				300,000,000
Short-term borrowings				0	$ 50,000,000
5-Year Facility [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity				2,500,000,000
364-Day Facility [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity				$ 1,000,000,000