Commercial Paper and Long-term Debt (Schedule of Notes Payable and Long-term Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Long-term Debt [Line Items]
Total debt issued and outstanding	$ 4,760	$ 4,755
$500 notes due 2020, 4.40%
Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.40%
Total debt issued and outstanding	$ 500	499
$1,000 notes due 2020, 3.125%
Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	3.125%
Total debt issued and outstanding	$ 999	998
$1,100 notes due 2022, 2.50%
Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	2.50%
Total debt issued and outstanding	$ 1,097	1,096
$300 notes due 2024, 3.15%
Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	3.15%
Total debt issued and outstanding	$ 298	298
$382 notes due 2027, 7.20%
Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	7.20%
Total debt issued and outstanding	$ 374	373
$185 notes due 2028, 7.00%
Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	7.00%
Total debt issued and outstanding	$ 185	185
$600 notes due 2040, 4.875%
Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.875%
Total debt issued and outstanding	$ 592	592
$425 notes due 2041, 4.70%
Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.70%
Total debt issued and outstanding	$ 420	419
$300 notes due 2044, 4.20%
Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.20%
Total debt issued and outstanding	$ 295	$ 295