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Fair Value - Recurring Fair Value Measures (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011
Liabilities [Abstract] |  |
Fair Value Activity in Level 3 | $ 0 |
Fair Value Hierarchy Transfers, All Levels | 0 | 0
Recurring Fair Value Measures [Member] |  |
Investments [Abstract] |  |
Available-for-sale Securities - Equity Securities | 18.0 | 17.6
Available-for-sale Securities - Corporate Debt Securities | 8.8 | 8.2
Cash Equivalents | 20.7 | 11.4
Total Fair Value of Assets | 47.5 | 37.2
Liabilities [Abstract] |  |
Deferred Compensation | 14.0 | 12.8
Derivatives - Interest Rate Swap | 0.7 | 0.4
Total Fair Value of Liabilities | 14.7 | 13.2
Total Net Fair Value of Assets (Liabilities) | 32.8 | 24.0
Recurring Fair Value Measures [Member] | Level 1 [Member] |  |
Investments [Abstract] |  |
Available-for-sale Securities - Equity Securities | 18.0 | 17.6
Available-for-sale Securities - Corporate Debt Securities | 0 | 0
Cash Equivalents | 20.7 | 11.4
Total Fair Value of Assets | 38.7 | 29.0
Liabilities [Abstract] |  |
Deferred Compensation | 0 | 0
Derivatives - Interest Rate Swap | 0 | 0
Total Fair Value of Liabilities | 0 | 0
Total Net Fair Value of Assets (Liabilities) | 38.7 | 29.0
Recurring Fair Value Measures [Member] | Level 2 [Member] |  |
Investments [Abstract] |  |
Available-for-sale Securities - Equity Securities | 0 | 0
Available-for-sale Securities - Corporate Debt Securities | 8.8 | 8.2
Cash Equivalents | 0 | 0
Total Fair Value of Assets | 8.8 | 8.2
Liabilities [Abstract] |  |
Deferred Compensation | 14.0 | 12.8
Derivatives - Interest Rate Swap | 0.7 | 0.4
Total Fair Value of Liabilities | 14.7 | 13.2
Total Net Fair Value of Assets (Liabilities) | (5.9) | (5.0)
Recurring Fair Value Measures [Member] | Level 3 [Member] |  |
Investments [Abstract] |  |
Available-for-sale Securities - Equity Securities | 0 | 0
Available-for-sale Securities - Corporate Debt Securities | 0 | 0
Cash Equivalents | 0 | 0
Total Fair Value of Assets | 0 | 0
Liabilities [Abstract] |  |
Deferred Compensation | 0 | 0
Derivatives - Interest Rate Swap | 0 | 0
Total Fair Value of Liabilities | 0 | 0
Total Net Fair Value of Assets (Liabilities) | $ 0 | $ 0

X
- Definition This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. + References No definition available. + Details
Name: | ale_AvailableForSaleSecuritiesFairValueDisclosureDebtSecurities
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represeent Available-for-sale Securities which consist of all investments in certain equity securities neither classified as trading or held-to-maturity securities. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options and put options, but do not include convertible debt. + References No definition available. + Details
Name: | ale_AvailableForSaleSecuritiesFairValueDisclosureEquitySecurities
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This element represents the portion of the balance sheet assertion, Deferred Compensation, valued at fair value by the entity. + References No definition available. + Details
Name: | ale_DeferredCompensationFairValueDisclosure
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Fair Value Activity in Level 3 + References No definition available. + Details
Name: | ale_FairValueActivityInLevel3
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The amount of transfers in and out of levels 1, 2 and 3 of the fair value hierarchy during the period. + References No definition available. + Details
Name: | ale_FairValueHierarchyTransfersAllLevels
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition This element represents the portion of the balance sheet Investments line item's cash and cash equivalents, valued at fair value by the entity. + References No definition available. + Details
Name: | ale_InvestmentsCashAndCashEquivalentsFairValueDisclosure
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This element represents the aggregate of the assets and liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Derivative instrument obligations meeting the definition of a liability which the Company is a party to as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 15 -Paragraph 83 -URI http://asc.fasb.org/extlink&oid=6913464&loc=d3e34841-113949 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 10 -Subparagraph i -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 133 -Paragraph 6 -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
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- Details
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Period Type: | duration

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- Definition This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
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