Jointly Owned Facilities (Detail) - PPL Energy Supply LLC [Member] - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Jointly Owned Facilities [Line Items]
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	$ 4,746	$ 4,686
Accumulated depreciation	3,591	3,545
Construction work in progress	$ 117	$ 76
Conemaugh Generation Plant [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	16.25%	16.25%
Electric plant	$ 330	$ 247
Accumulated depreciation	141	131
Construction work in progress	$ 2	$ 63
Keystone Generation Plant [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	12.34%	12.34%
Electric plant	$ 213	$ 207
Accumulated depreciation	102	91
Construction work in progress	$ 2	$ 2
Merrill Creek Reservoir [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	8.37%	8.37%
Other property	$ 22
Accumulated depreciation	$ 15	$ 22
Construction work in progress		$ 16
Colstrip Units 1 and 2 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	50.00%	50.00%
Colstrip Unit 3 [Member]
Jointly Owned Facilities [Line Items]
Ownership interest	30.00%	30.00%

A seperate entity's proportionate share of interest in a utility plant which the company shares no ownership.

No definition available.

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- Definition

The proportionate share of operating and construction costs each party is responsible for in a jointly owned electricity generation plant.

+ References

No definition available.

+ Details

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- Definition

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 10.C)

-URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section C

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

The period end amount of construction work in progress at a jointly owned electricity generation plant.

+ References

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- Definition

The period end ownership amount of other assets in a jointly owned utility plant.

+ References

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- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

+ References

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- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

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