Debt - Short-term and Long-term Debt (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Debt Instrument [Line Items]
Total short-term debt		$ 141		$ 276
Long-term debt, carrying value		5,207		4,756
Repayment of long-term debt		1,250	$ 0
Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Current maturities of long-term debt		131		250
Other, including capital leases		10		26
Total short-term debt		141		276
Long-term debt, carrying value		5,207		4,756
$131 million, 6.35% Notes due 2018 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value		$ 131		$ 131
Long-term debt, stated interest rate		6.35%		6.35%
Long-term debt, carrying value		$ 0		$ 131
$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		$ 252		$ 252
$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		$ 301		$ 301
$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		$ 301		$ 302
$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		$ 745		$ 744
$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		$ 893
$600 million, 3.05% Notes due 2027 | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value		$ 600		$ 0
Long-term debt, stated interest rate		3.05%		3.05%
Long-term debt, carrying value		$ 594		$ 0
$259 million, 7.875% Debentures due 2027 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value		$ 259		$ 300
Long-term debt, stated interest rate		7.875%		7.875%
Long-term debt, carrying value		$ 258		$ 299
$45 million, 8.3% Step Down Notes due 2033 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value		$ 45		$ 83
Long-term debt, stated interest rate		8.30%		8.30%
Long-term debt, carrying value		$ 45		$ 82
$191 million, 6.15% Notes due 2036 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value		$ 191		$ 500
Long-term debt, stated interest rate		6.15%		6.15%
Long-term debt, carrying value		$ 190		$ 498
$121 million, 5.875% Notes due 2041 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value		$ 121		$ 300
Long-term debt, stated interest rate		5.875%		5.875%
Long-term debt, carrying value		$ 119		$ 296
$317 million, 5.375% Notes due 2042 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value		$ 317		$ 750
Long-term debt, stated interest rate		5.375%		5.375%
Long-term debt, carrying value		$ 314		$ 743
$1,000 million, 3.875% Notes due 2047 | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value		$ 1,000		$ 0
Long-term debt, stated interest rate		3.875%		3.875%
Long-term debt, carrying value		$ 987		$ 0
Other, including capital leases [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value		$ 12		$ 20
$250 million, 5.375% Notes due 2017 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Repayment of long-term debt	$ 250