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Accumulated Other Comprehensive Income - Reclassification of Component of Accumulated Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other, net									$ (29)us-gaap_OtherNonoperatingIncomeExpense	$ (74)us-gaap_OtherNonoperatingIncomeExpense	$ (18)us-gaap_OtherNonoperatingIncomeExpense
Operating revenues	3,437us-gaap_Revenues	3,602us-gaap_Revenues	3,561us-gaap_Revenues	3,396us-gaap_Revenues	3,500us-gaap_Revenues	3,621us-gaap_Revenues	3,526us-gaap_Revenues	3,336us-gaap_Revenues	13,996us-gaap_Revenues	13,983us-gaap_Revenues	13,649us-gaap_Revenues
Gains and losses on cash flow hedges, Tax (expense) benefit									(413)us-gaap_IncomeTaxExpenseBenefit	(364)us-gaap_IncomeTaxExpenseBenefit	(443)us-gaap_IncomeTaxExpenseBenefit
Consolidated net income	598us-gaap_ProfitLoss	281us-gaap_ProfitLoss	222us-gaap_ProfitLoss	237us-gaap_ProfitLoss	(599)us-gaap_ProfitLoss	297us-gaap_ProfitLoss	256us-gaap_ProfitLoss	176us-gaap_ProfitLoss	1,338us-gaap_ProfitLoss	130us-gaap_ProfitLoss	860us-gaap_ProfitLoss
Amount Reclassified from Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gains and losses on cash flow hedges, Total before tax									8	3	(15)
Gains and losses on cash flow hedges, Tax (expense) benefit									(3)	(1)	5
Consolidated net income									5	2	(10)
Interest Rate Swaps [Member] \| Amount Reclassified from Accumulated Other Comprehensive Income [Member] \| Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense									(10)	(7)	(3)
Treasury Rate Locks [Member] \| Amount Reclassified from Accumulated Other Comprehensive Income [Member] \| Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense									(1)	(2)	(7)
Foreign Currency Derivatives [Member] \| Amount Reclassified from Accumulated Other Comprehensive Income [Member] \| Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other, net									27	21	(15)
Electricity Commodity Derivatives [Member] \| Amount Reclassified from Accumulated Other Comprehensive Income [Member] \| Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating revenues									$ (8)	$ (9)	$ 10