Fair Value Measurements (Details) - Derivative Assets and Liabilities - Natural Gas and Interest Rate Derivatives [Member] - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	$ 165	$ 284	$ 110
Liabilities	(61)	(93)	(47)
Netting of counterparty offset and cash collateral, Assets	33	52	49
Netting of counterparty offset and cash collateral, Liabilities	56	81	76
Quoted prices in active markets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	40	58	4
Liabilities	(57)	(80)	(72)
Significant other observable inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	92	174	57
Liabilities	$ (60)	$ (94)	$ (51)