Jointly Owned Facilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Jointly Owned Utility Plant Interests [Line Items]
Percentage of leasehold interest in Colstrip Units 1 and 2	50.00%
Percentage of leasehold interest in Colstrip Unit 3	30.00%
Percentage of leasehold interest by North Western in Colstrip Unit 4	30.00%
Proportionate share by each party of operating and construction costs regarding Colstrip Unit 3 or 4	15.00%
Susquehanna Nuclear Generation Plant [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership interest	90.00%	90.00%
Electric plant	$ 4,608	$ 4,553
Accumulated depreciation	3,496	3,487
Construction work in progress	42	79
Conemaugh Generation Plant [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership interest	16.25%	16.25%
Electric plant	233	213
Accumulated depreciation	115	106
Construction work in progress	14	11
Keystone Generation Plant [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership interest	12.34%	12.34%
Electric plant	198	196
Accumulated depreciation	69	60
Construction work in progress	3	2
Trimble County-Unit 1 and 2 Generation Plant [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership interest	75.00%	75.00%
Electric plant	1,245	352
Accumulated depreciation	61	10
Construction work in progress	35	907
Merrill Creek Reservoir [Member]
Jointly Owned Utility Plant Interests [Line Items]
Ownership interest	8.37%	8.37%
Other property	22	22
Accumulated depreciation	$ 15	$ 15