Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Financial Assets:
Financial Assets: Natural Gas Price Swaps	$ 4
Debt Disclosure [Abstract]
Estimated Fair Value of Debt	7,163	$ 6,524
Aggregate Principal Amount of Current and Long-Term Debt	6,990	6,390
Fair Value, Inputs, Level 1 [Member]
Financial Assets:
Financial Assets: Natural Gas Price Swaps	0	0
Fair Value, Inputs, Level 2 [Member]
Financial Assets:
Financial Assets: Natural Gas Price Swaps	4	0
Fair Value, Inputs, Level 3 [Member]
Financial Assets:
Financial Assets: Natural Gas Price Swaps	$ 0	$ 0