Debt - Short-term and Long-term Debt (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Total short-term debt	$ 109		$ 240
Long-term debt, carrying value	5,195		5,199
Repayment of long-term debt	131	$ 250
Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Commercial paper	100		100
Current maturities of long-term debt	0		131
Other, including capital leases	9		9
Total short-term debt	109		240
Long-term debt, carrying value	5,195		5,199
$250 million, 4.375% Notes due 2020 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 250		$ 250
Long-term debt, stated interest rate	4.375%		4.375%
Long-term debt, carrying value	$ 247		$ 249
$300 million, 5.125% Notes due 2020 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 300		$ 300
Long-term debt, stated interest rate	5.125%		5.125%
Long-term debt, carrying value	$ 297		$ 299
$78 million, 6.37% Notes due 2021 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 78		$ 78
Long-term debt, stated interest rate	6.37%		6.37%
Long-term debt, carrying value	$ 78		$ 78
$300 million, 4.5% Notes due 2021 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 300		$ 300
Long-term debt, stated interest rate	4.50%		4.50%
Long-term debt, carrying value	$ 296		$ 299
$750 million, 4% Notes due 2022 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 750		$ 750
Long-term debt, stated interest rate	4.00%		4.00%
Long-term debt, carrying value	$ 746		$ 745
$100 million, 7.65% Notes due 2023 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 100		$ 100
Long-term debt, stated interest rate	7.65%		7.65%
Long-term debt, carrying value	$ 100		$ 100
$17 million, 8.3% Notes due 2023 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 17		$ 17
Long-term debt, stated interest rate	8.30%		8.30%
Long-term debt, carrying value	$ 17		$ 17
$900 million, 3.25% Notes Due 2025 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 900		$ 900
Long-term debt, stated interest rate	3.25%		3.25%
Long-term debt, carrying value	$ 894		$ 894
$600 million, 3.05% Notes due 2027 | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 600		$ 600
Long-term debt, stated interest rate	3.05%		3.05%
Long-term debt, carrying value	$ 594		$ 594
$259 million, 7.875% Debentures due 2027 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 259		$ 259
Long-term debt, stated interest rate	7.875%		7.875%
Long-term debt, carrying value	$ 258		$ 258
$45 million, 8.3% Step Down Notes due 2033 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 45		$ 45
Long-term debt, stated interest rate	8.30%		8.30%
Long-term debt, carrying value	$ 45		$ 45
$191 million, 6.15% Notes due 2036 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 191		$ 191
Long-term debt, stated interest rate	6.15%		6.15%
Long-term debt, carrying value	$ 190		$ 190
$121 million, 5.875% Notes due 2041 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 121		$ 121
Long-term debt, stated interest rate	5.875%		5.875%
Long-term debt, carrying value	$ 119		$ 119
$317 million, 5.375% Notes due 2042 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 317		$ 317
Long-term debt, stated interest rate	5.375%		5.375%
Long-term debt, carrying value	$ 315		$ 315
$1,000 million, 3.875% Notes due 2047 | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 1,000		$ 1,000
Long-term debt, stated interest rate	3.875%		3.875%
Long-term debt, carrying value	$ 988		$ 988
Other, including capital leases [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 11		$ 9