Debt (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Face value of debt	$ 19,230,000,000	$ 19,640,000,000
Gain (loss) on early extinguishment of debt	0	18,000,000	$ (38,000,000)
4.20% Senior Notes due 2029 [Member]
Line of Credit Facility [Line Items]
Face value of debt	$ 500,000,000
Stated interest rate	4.20%
Debt Due August 2019 [Member]
Line of Credit Facility [Line Items]
Stated interest rate	2.30%
Redemption and repayment of senior notes, face value	$ 600,000,000
5.625% Senior Notes due 2019 [Member]
Line of Credit Facility [Line Items]
Stated interest rate	5.625%
Debt redemptions and repayments, cash	$ 220,000,000
2.750% Senior Notes due 2019 [Member]
Line of Credit Facility [Line Items]
Stated interest rate	2.75%
Debt redemptions and repayments, cash	$ 90,000,000
5.875% Junior Subordinated Debentures due 2057 [Member]
Line of Credit Facility [Line Items]
Stated interest rate	5.875%
5.875% Junior Subordinated Debentures due 2057 [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	3.895%
6.25% Junior Subordinated Debentures due 2057 [Member]
Line of Credit Facility [Line Items]
Stated interest rate	6.25%
6.25% Junior Subordinated Debentures due 2057 [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	3.899%
2.250% Senior Notes due 2022 [Member]
Line of Credit Facility [Line Items]
Stated interest rate	2.25%
Outstanding principal	$ 50,000,000
3.45% Senior Notes due 2026 [Member]
Line of Credit Facility [Line Items]
Stated interest rate	3.45%
Outstanding principal	$ 124,000,000
Junior Debentures And Senior Notes [Member]
Line of Credit Facility [Line Items]
Face value of debt			3,100,000,000
Senior Notes [Member]
Line of Credit Facility [Line Items]
Potential change	0.25%
Debt redemptions and repayments, cash		1,100,000,000
Redemption and repayment of senior notes, face value		1,130,000,000	4,670,000,000
Debt redeemed prior to maturity, face value			4,270,000,000
Gain (loss) on early extinguishment of debt		18,000,000	(38,000,000)
Gain (loss) on early extinguishment of debt. net of tax		$ 14,000,000	$ (21,000,000)
Senior Notes [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Potential change	2.00%