Debt - Short-term and Long-term Debt (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 14, 2017
Debt Instrument [Line Items]
Commercial paper		$ 100	$ 0
Current maturities of long-term debt		131	250
Other, including capital leases		9	26
Total short-term debt		240	276
Long-term debt, carrying value		5,199	4,756
Repayment of long-term debt	$ 250	1,250	0	$ 851
$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		$ 249	$ 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		$ 299	$ 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		$ 299	$ 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
Long-term debt, stated interest rate		3.05%
Long-term debt, carrying value		$ 594	0		$ 594
$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		$ 315	$ 743
$1,000 million, 3.875% Notes due 2047 | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value		$ 1,000
Long-term debt, stated interest rate		3.875%
Long-term debt, carrying value		$ 988	0		$ 987
Other, including capital leases [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value		$ 9	$ 20