Debt - Short-term and Long-term Debt (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Disclosure [Abstract]
Commercial paper	$ 100	$ 100
Other	47	133
Total short-term debt	147	233
Debt Instrument [Line Items]
Long-term debt, carrying value	5,005	5,014
$600 million, 2.75% Notes due 2016 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	600	600
Long-term debt, stated interest rate	2.75%	2.75%
Long-term debt, carrying value	600	600
$250 million, 5.375% Notes due 2017 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	250	250
Long-term debt, stated interest rate	5.375%	5.375%
Long-term debt, carrying value	250	250
$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	131	131
$251 million, 8.5% Notes due 2019 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	251	251
Long-term debt, stated interest rate	8.50%	8.50%
Long-term debt, carrying value	251	251
$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	254	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	303	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	303	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	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
$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	300	300
$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	83	83
$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	500	500
$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	298	298
$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	750	750
Other Long Term Debt [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 42	$ 65