The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References+ Details

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14326-108349

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 5

-Paragraph 9

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 5

-Paragraph 10

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 4

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14435-108349

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14557-108349

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 5

-Paragraph 11

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 5

-Paragraph 12

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Litigation (Details) (USD $)					0 Months Ended
Litigation (Details) (USD $)	Sep. 30, 2013 Colstrip Units 1 and 2 [Member]	Sep. 30, 2013 Colstrip Units 3 and 4 [Member]	Sep. 30, 2013 Pending Litigation [Member]	Dec. 31, 2012 Pending Litigation [Member]	Mar. 06, 2013 Pending Litigation [Member] Sierra Club and Montana Environmental Information Center [Member]	Mar. 06, 2013 Pending Litigation [Member] Clean Air Act - Prevention of Significant Deterioration (PSD) Violations Sierra Club and Montana Environmental Information Center [Member] claim	Mar. 06, 2013 Pending Litigation [Member] Failure to Update Title V Permit [Member] Sierra Club and Montana Environmental Information Center [Member] claim	Mar. 06, 2013 Pending Litigation [Member] Major Modifications to Title V Permit [Member] Sierra Club and Montana Environmental Information Center [Member] claim
Loss Contingencies [Line Items]
Ownership interest (percent)	50.00%	25.00%
Numbe of claims						39	1	36
Damages sought, civil penalties to be used for beneficial mitigation projects					$ 100,000
Litigation claims accrual			$ 1,400,000	$ 3,400,000