Derivative Instruments and Hedging Activities (Cash Flow Hedges Included in Accumulated Other Comprehensive Loss) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated Other Comprehensive Loss After-tax	$ 21		$ 50
2012 Portion Expected to be Reclassified to Income during the Next 12 Months	11		29
Energy Commodity [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated Other Comprehensive Loss After-tax	11	[1]	40	[1]
2012 Portion Expected to be Reclassified to Income during the Next 12 Months	10	[1]	28	[1]
Maximum Term	20 months	[1]	32 months	[1]
Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated Other Comprehensive Loss After-tax	10		10
2012 Portion Expected to be Reclassified to Income during the Next 12 Months	$ 1		$ 1
Maximum Term	239 months		251 months

[1] The unrealized derivative losses recorded in AOCL relate to forecasted physical natural gas and electricity purchases which are used to supply retail natural gas and electricity contracts that are in gain positions and subject to accrual accounting. Under accrual accounting, no asset is recorded on PHI's consolidated balance sheet and the purchase cost is not recognized until the period of distribution.