Jointly Owned Facilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Jointly Owned Facilities [Line Items]
Percentage of leasehold interest in Colstrip Units 1 and 2	50.00%	50.00%
Percentage of leasehold interest in Colstrip Unit 3	30.00%	30.00%
Percentage of interest by Northwestern in Colstrip Unit 4	30.00%	30.00%
Proportionate share by each party of operating and construction costs regarding Colstrip Unit 3 or 4	15.00%	15.00%
Susquehanna Nuclear Generation Plant [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	90.00%	90.00%
Electric plant	4,628	4,608
Accumulated depreciation	3,530	3,496
Construction work in progress	65	42
Conemaugh Generation Plant [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	16.25%	16.25%
Electric plant	238	233
Accumulated depreciation	122	115
Construction work in progress	30	14
Keystone Generation Plant [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	12.34%	12.34%
Electric plant	206	198
Accumulated depreciation	82	69
Construction work in progress	3	3
Trimble County-Units 1 and 2 Generation Plant [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	75.00%	75.00%
Electric plant	1,279	1,245
Accumulated depreciation	112	61
Construction work in progress	43	35
Merrill Creek Reservoir [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	8.37%	8.37%
Other property	22	22
Accumulated depreciation	15	15
PPL Energy Supply LLC [Member]
Jointly Owned Facilities [Line Items]
Percentage of leasehold interest in Colstrip Units 1 and 2	50.00%	50.00%
Percentage of leasehold interest in Colstrip Unit 3	30.00%	30.00%
Percentage of interest by Northwestern in Colstrip Unit 4	30.00%	30.00%
Proportionate share by each party of operating and construction costs regarding Colstrip Unit 3 or 4	15.00%	15.00%
PPL Energy Supply LLC [Member] | Susquehanna Nuclear Generation Plant [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	90.00%	90.00%
Electric plant	4,628	4,608
Accumulated depreciation	3,530	3,496
Construction work in progress	65	42
PPL Energy Supply LLC [Member] | Conemaugh Generation Plant [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	16.25%	16.25%
Electric plant	238	233
Accumulated depreciation	122	115
Construction work in progress	30	14
PPL Energy Supply LLC [Member] | Keystone Generation Plant [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	12.34%	12.34%
Electric plant	206	198
Accumulated depreciation	82	69
Construction work in progress	3	3
PPL Energy Supply LLC [Member] | Merrill Creek Reservoir [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	8.37%	8.37%
Other property	22	22
Accumulated depreciation	15	15
LG And E And KU Energy LLC [Member] | Trimble County Unit 1 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	75.00%	75.00%
Electric plant	304	297
Other property
Accumulated depreciation	33	19
Construction work in progress	10	11
LG And E And KU Energy LLC [Member] | Trimble County Unit 2 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	75.00%	75.00%
Electric plant	975	948
Other property
Accumulated depreciation	79	42
Construction work in progress	33	24
Louisville Gas And Electric Co [Member] | E W Brown Units 6 And 7 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	38.00%	38.00%
Electric plant	40	39
Other property
Accumulated depreciation	5	3
Construction work in progress
Louisville Gas And Electric Co [Member] | Paddys Run Unit 13 And E W Brown Unit 5 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	53.00%	53.00%
Electric plant	46	44
Other property
Accumulated depreciation	3	2
Construction work in progress		5
Louisville Gas And Electric Co [Member] | Trimble County Unit 1 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	75.00%	75.00%
Electric plant	304	297
Other property
Accumulated depreciation	33	19
Construction work in progress	10	11
Louisville Gas And Electric Co [Member] | Trimble County Unit 2 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	14.25%	14.25%
Electric plant	198	190
Other property
Accumulated depreciation	14	7
Construction work in progress	13	7
Louisville Gas And Electric Co [Member] | Trimble County Units 5 And 6 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	29.00%	29.00%
Electric plant	29	31
Other property
Accumulated depreciation	2	1
Construction work in progress
Louisville Gas And Electric Co [Member] | Trimble County Units 7 Through 10 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	37.00%	37.00%
Electric plant	68	64
Other property
Accumulated depreciation	6	4
Construction work in progress	2	1
Louisville Gas And Electric Co [Member] | Cane Run Unit 7 Combined Cycle Gas Turbine [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	22.00%
Electric plant
Other property
Accumulated depreciation
Construction work in progress	16
Kentucky Utilities Co [Member] | E W Brown Units 6 And 7 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	62.00%	62.00%
Electric plant	64	64
Other property
Accumulated depreciation	7	5
Construction work in progress	1
Kentucky Utilities Co [Member] | Paddys Run Unit 13 And E W Brown Unit 5 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	47.00%	47.00%
Electric plant	42	39
Other property
Accumulated depreciation	2	2
Construction work in progress		4
Kentucky Utilities Co [Member] | Trimble County Unit 2 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	60.75%	60.75%
Electric plant	777	758
Other property
Accumulated depreciation	65	35
Construction work in progress	20	17
Kentucky Utilities Co [Member] | Trimble County Units 5 And 6 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	71.00%	71.00%
Electric plant	70	66
Other property
Accumulated depreciation	4	2
Construction work in progress		4
Kentucky Utilities Co [Member] | Trimble County Units 7 Through 10 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	63.00%	63.00%
Electric plant	116	109
Other property
Accumulated depreciation	10	6
Construction work in progress	2	5
Kentucky Utilities Co [Member] | Cane Run Unit 7 Combined Cycle Gas Turbine [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	78.00%
Electric plant
Other property
Accumulated depreciation
Construction work in progress	53