Leases (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating leases rent expense
Operating leases, rent expense	$ 42	$ 39	$ 37
Buildings
Capital leased assets, gross	267	267
Accumulated amortization	(56)	(46)
Total	211	221
Capital lease expense included in O&M or purchased power expense	25	25	26
Capital leases minimum annual payments
2012	28
2013	36
2014	26
2015	26
2016	25
Thereafter	201
Minimum annual payments, total	342
Less amount representing imputed interest	(131)
Present value of net minimum lease payments under capital leases	211
Operating leases minimum annual payments
2012	61
2013	85
2014	82
2015	79
2016	79
Thereafter	791
Minimum annual payments, total	1,177
Purchased power
Operating leases rent expense
Operating leases, rent expense	62	61	11
Building Lease Expiring 2035
Operating leases rent expense
Operating leases, future rent expense	53
Additional lease information
Description of lessee leasing arrangements, operating leases	lease term expires July 2035 and provides for no rental payments during the last 15 years of the lease
Leases minimum rentals
Operating leases, rent expense, minimum rentals	7
PEC
Noncancelable leases, rents received
Noncancelable leases, rents received	35	33	34
Noncancelable leases, rents to be received	12
PEC | Purchased power
Operating leases rent expense
Operating leases, rent expense	62	38	11
Operating leases minimum annual payments
Minimum annual payments, total	512
Additional lease information
Long-term contract for purchase of electric power, description	expire through 2032
Long-term contract for purchase of electric power, MW	576
Long-term contract for purchase of electric power, share of plant output being purchased	100.00%
PEF
Noncancelable leases, rents received
Noncancelable leases, rents received	86	85	84
PEF | Purchased power
Operating leases rent expense
Operating leases, rent expense		23
Operating leases minimum annual payments
Minimum annual payments, total	421
Additional lease information
Long-term contract for purchase of electric power, description	minimum annual payments beginning in June 2012 and expires in 2027
Long-term contract for purchase of electric power, MW	640
Long-term contract for purchase of electric power, share of plant output being purchased	100.00%
PEF | Building Lease Expiring 2047
Buildings
Future capital lease expense to be included in O&M or purchased power expense	51
Capital leases minimum annual payments
Minimum annual payments, total	103
Additional lease information
Description of lessee leasing arrangements, capital leases	minimum annual payments from 2007 through 2026 and no payments from 2027 through 2047
Leases minimum rentals
Capital leases, future minimum payments due, annual	5
PEF | Tolling Agreement 517 MW | Purchased power
Capital leases minimum annual payments
Minimum annual payments, total	348
Additional lease information
Description of lessee leasing arrangements, capital leases	2007 through 2024
Long-term contract for purchase of electric power, MW	517
Long-term contract for purchase of electric power, share of plant output being purchased	100.00%
Leases minimum rentals
Capital leases, future minimum payments due, annual	$ 21