Long-term Debt Narrative (Details) - Senior Notes [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Senior Notes, 4.50% due 2022 [Member]
Debt Instrument [Line Items]
Stated interest rate on debt	4.50%	4.50%
Senior Notes, 5.125% due 2020 [Member]
Debt Instrument [Line Items]
Stated interest rate on debt	5.125%
Senior Notes, 5.95% due 2042 [Member]
Debt Instrument [Line Items]
Stated interest rate on debt	5.95%	5.95%
Senior Notes, 6.50% due 2036 [Member]
Debt Instrument [Line Items]
Stated interest rate on debt	6.50%	6.50%
Senior Notes, 6.75% due 2040 [Member]
Debt Instrument [Line Items]
Stated interest rate on debt	6.75%
Senior Notes, 6.80% due 2037 [Member]
Debt Instrument [Line Items]
Stated interest rate on debt	6.80%
Senior Notes, 7.00% due 2038 [Member]
Debt Instrument [Line Items]
Stated interest rate on debt	7.00%
Senior Notes [Member]
Debt Instrument [Line Items]
Book Value of Debt Repurchase	$ 527	$ 138
Gain on repurchase of debt	$ 84	$ 11