Acquisitions - Chanarambie/Viking (Details) (Chanarambie/Viking [Member], Subsequent Event [Member], USD $) In Millions, unless otherwise specified	0 Months Ended
Apr. 15, 2015	Apr. 15, 2015 MW
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Apr. 15, 2015
Business Acquisition, Name of Acquired Entity	Chanarambie/Viking
Payments to Acquire Businesses, Gross	$ 47.5
Generating Capacity (MW)	97.5ale_GeneratingCapacityMw	97.5ale_GeneratingCapacityMw
Chanarambie/Viking PPA (expires 2018) [Member]
Business Acquisition [Line Items]
Generating Capacity (MW)	12	12
Chanarambie/Viking PPA (expires 2023) [Member]
Business Acquisition [Line Items]
Generating Capacity (MW)	85.5	85.5

Capacity of a generating facility, unit, or project in megawatts.

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- Definition

Date when the acquirer obtains control of the acquiree, in CCYY-MM-DD format.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 25

-Paragraph 6

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

-Publisher FASB

-Name Accounting Standards Codification

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

-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Name of the acquired entity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

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- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

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