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Restructuring (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Sep. 30, 2014
Sep. 30, 2013
Restructuring and Related Activities [Abstract]        
Expected increase (decrease) in operation and maintenance expense $ (500)tve_Expectedincreasedecreaseinoperationandmaintenanceexpense      
Restructuring and Related Cost, Number of Positions Eliminated     2,000us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminated  
Severance cost liability 4us-gaap_RestructuringReserve 12us-gaap_RestructuringReserve 45us-gaap_RestructuringReserve 0us-gaap_RestructuringReserve
Liabilities incurred during the period 0us-gaap_RestructuringCharges 12us-gaap_RestructuringCharges    
Actual costs paid during the period (40)us-gaap_PaymentsForRestructuring 0us-gaap_PaymentsForRestructuring    
Adjustments to estimate during the period $ (1)us-gaap_RestructuringReserveAccrualAdjustment $ 0us-gaap_RestructuringReserveAccrualAdjustment