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

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

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Litigation (Details) (USD $)	0 Months Ended
Litigation (Details) (USD $)	Mar. 06, 2013	Dec. 31, 2014	Dec. 31, 2013
Pending Litigation
Loss Contingencies [Line Items]
Litigation claims accrual		$ 1,700,000	$ 1,400,000
Colstrip Unit One and Two
Loss Contingencies [Line Items]
Equity Method Investment, Ownership Percentage		50.00%
Colstrip Unit Three and Four
Loss Contingencies [Line Items]
Equity Method Investment, Ownership Percentage		25.00%
Sierra Club and Montana Environmental Information Center [Member] \| Pending Litigation
Loss Contingencies [Line Items]
Loss Contingency, Damages Sought, Value	$ 100,000
Sierra Club and Montana Environmental Information Center [Member] \| Failure to Update Title V Permit \| Pending Litigation
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number	1
Sierra Club and Montana Environmental Information Center [Member] \| Title V compliance certifications incomplete \| Pending Litigation
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number	1
Sierra Club and Montana Environmental Information Center [Member] \| Opacity \| Pending Litigation
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number	1