Capital Structure - Total debt outstanding (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Nov. 19, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Due after one year through five years	$ 1,600,000,000		$ 1,000,000,000
Due after five years through ten years	1,150,000,000		550,000,000
Due after ten years	5,141,000,000		5,141,000,000
Total long-term debt principal	7,891,000,000		6,691,000,000
Debt issuance costs	(66,000,000)		(60,000,000)
Total long-term debt	7,825,000,000		6,631,000,000
Short-term debt	0		0
Total debt	$ 7,825,000,000		6,631,000,000
Redemption price as percentage of principal amount plus accrued and unpaid interest	100.00%
Floating Rate Senior Notes, due 2021
Debt Instrument [Line Items]
Due after one year through five years	$ 250,000,000		250,000,000
Interest rate spread over LIBOR (as a percent)	0.43%
Floating Rate Senior Notes, due 2023
Debt Instrument [Line Items]
Due after one year through five years	$ 250,000,000		250,000,000
Interest rate spread over LIBOR (as a percent)	0.63%
3.15% Senior Notes, due 2023
Debt Instrument [Line Items]
Due after one year through five years	$ 500,000,000		500,000,000
Note stated interest rate (as a percent)	3.15%
0.75% Senior Notes, due 2025
Debt Instrument [Line Items]
Due after one year through five years	$ 600,000,000		0
Total long-term debt		$ 600,000,000
Note stated interest rate (as a percent)	0.75%	0.75%
3.28% Senior Notes, due 2026
Debt Instrument [Line Items]
Due after five years through ten years	$ 550,000,000		550,000,000
Note stated interest rate (as a percent)	3.28%
1.45% Senior Notes, due 2030
Debt Instrument [Line Items]
Due after five years through ten years	$ 600,000,000		0
Total long-term debt		$ 600,000,000
Note stated interest rate (as a percent)	1.45%	1.45%
6.125% Senior Notes, due 2032
Debt Instrument [Line Items]
Due after ten years	$ 159,000,000		159,000,000
Note stated interest rate (as a percent)	6.125%
5.35% Senior Notes due 2033
Debt Instrument [Line Items]
Due after ten years	$ 323,000,000		323,000,000
Note stated interest rate (as a percent)	5.35%
5.55% Senior Notes due 2035
Debt Instrument [Line Items]
Due after ten years	$ 546,000,000		546,000,000
Note stated interest rate (as a percent)	5.55%
5.95% Senior Notes, due 2036
Debt Instrument [Line Items]
Due after ten years	$ 386,000,000		386,000,000
Note stated interest rate (as a percent)	5.95%
6.900% Senior Debentures, due 2038
Debt Instrument [Line Items]
Due after ten years	$ 165,000,000		165,000,000
Note stated interest rate (as a percent)	6.90%
5.20% Senior Notes due 2042
Debt Instrument [Line Items]
Due after ten years	$ 62,000,000		62,000,000
Note stated interest rate (as a percent)	5.20%
4.50% Senior Notes, due 2043
Debt Instrument [Line Items]
Due after ten years	$ 500,000,000		500,000,000
Note stated interest rate (as a percent)	4.50%
4.20% Senior Notes, due 2046
Debt Instrument [Line Items]
Due after ten years	$ 700,000,000		700,000,000
Note stated interest rate (as a percent)	4.20%
3.85% Senior Notes, due 2049
Debt Instrument [Line Items]
Due after ten years	$ 500,000,000		500,000,000
Note stated interest rate (as a percent)	3.85%
5.100% Subordinated Debentures, due 2053
Debt Instrument [Line Items]
Due after ten years	$ 500,000,000		500,000,000
Note stated interest rate (as a percent)	5.10%
5.750% Subordinated Debentures, due 2053
Debt Instrument [Line Items]
Due after ten years	$ 800,000,000		800,000,000
Note stated interest rate (as a percent)	5.75%
6.500% Junior Subordinated Debentures, due 2067
Debt Instrument [Line Items]
Due after ten years	$ 500,000,000		$ 500,000,000
Total long-term debt	$ 500,000,000
Note stated interest rate (as a percent)	6.50%