Debt - Short-term and Long-term Debt (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Debt Instrument [Line Items]
Total short-term debt	$ 141		$ 276
Long-term debt, carrying value	4,622		4,756
Repayment of long-term debt	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	4,622		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	$ 302		$ 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	$ 302		$ 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	$ 744		$ 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
$300 million, 7.875% Debentures due 2027 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 300		$ 300
Long-term debt, stated interest rate	7.875%		7.875%
Long-term debt, carrying value	$ 299		$ 299
$83 million, 8.3% Step Down Notes due 2033 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 83		$ 83
Long-term debt, stated interest rate	8.30%		8.30%
Long-term debt, carrying value	$ 82		$ 82
$500 million, 6.15% Notes due 2036 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 500		$ 500
Long-term debt, stated interest rate	6.15%		6.15%
Long-term debt, carrying value	$ 498		$ 498
$300 million, 5.875% Notes due 2041 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 300		$ 300
Long-term debt, stated interest rate	5.875%		5.875%
Long-term debt, carrying value	$ 296		$ 296
$750 million, 5.375% Notes due 2042 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 750		$ 750
Long-term debt, stated interest rate	5.375%		5.375%
Long-term debt, carrying value	$ 743		$ 743
Other, including capital leases [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 15		$ 20