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Debt (Schedule of Long-term Debt Instruments) (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Jun. 30, 2006
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 3,597,000,000	$ 3,003,000,000
Debt Issuance Cost	24,000,000	22,000,000
Debt Instrument, Face Amount			$ 850,000,000
Long-term debt	3,573,000,000	2,981,000,000
Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage	6.45%
Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 400,000,000	400,000,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage	5.00%
Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 399,000,000	399,000,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.70%
Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 400,000,000	400,000,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.95%
Senior Notes Due 2029 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 599,000,000	599,000,000
Debt Instrument, Face Amount	$ 600,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.90%
Subordinated Debentures Due 2042 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 400,000,000	400,000,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage	6.20%
Subordinated Debentures Due 2056 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 400,000,000	400,000,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage	5.75%
Junior Subordinated Debentures Due 2065 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 319,000,000	319,000,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.82%
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 83,000,000	86,000,000
Debt Instrument, Face Amount	$ 100,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.09%
Senior Notes Due 2030
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 597,000,000	$ 0
Debt Instrument, Face Amount	$ 600,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.15%