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Short-Term and Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Short-Term and Long-Term Debt [Abstract]
Short-Term Debt Outstanding	$ 7.1	$ 6.5
Long-Term Debt Outstanding	$ 856.5	$ 857.9
Debt Instrument, Required Indebtedness to Total Capital Ratio	0.65 to 1.00
Debt Instrument, Actual Indebtedness to Total Capital Ratio	0.43 to 1.00

X

- Definition

Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant.

+ References

No definition available.

+ Details

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Namespace Prefix:	ale_
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Balance Type:	na
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X

- Definition

Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant.

+ References

No definition available.

+ Details

Name:	ale_DebtInstrumentRequiredIndebtednessToTotalCapitalRatio
Namespace Prefix:	ale_
Data Type:	xbrli:stringItemType
Balance Type:	na
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X

- Details

Name:	ale_ShortTermAndLongTermDebtAbstract
Namespace Prefix:	ale_
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Period Type:	duration

X

- Definition

Carrying value as of the balance sheet date of the sum of short-term debt and current maturities of long-term debt and capital lease obligations, which are due within one year (or one business cycle if longer).

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Article 5



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19,20)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Sum of the carrying values as of the balance sheet date of all long-term debt, which is debt initially having maturities due after one year from the balance sheet date or beyond the operating cycle, if longer, but excluding the portions thereof scheduled to be repaid within one year (current maturities) or the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant