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Other Income (Expense) (Tables) | 9 Months Ended
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Sep. 30, 2015
Other Income (Expense) [Abstract] |
Other Income (Expense) [Table Text Block] |
|  | Quarter Ended |  |  | Nine Months Ended
|  | September 30, |  |  | September 30,
|  | 2015 |  | 2014 |  | 2015 |  | 2014
Millions |  |  |  |  |  |  |  |
AFUDC–Equity |  |  | $1.0 |  |  |  | $2.1 |  |  |  | $2.6 |  |  |  | $5.9 |
Gain on Sale of Available-for-sale Securities |  | — |  |  | — |  |  | 0.1 |  |  | 0.2 |
Investments and Other Income (Expense) |  | 0.7 |  |  | — |  |  | 0.8 |  |  | (0.1 | )
Total Other Income |  |  | $1.7 |  |  |  | $2.1 |  |  |  | $3.5 |  |  |  | $6.0 |

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- Definition Other Income (Expense) [Abstract] + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Tabular disclosure of the components of non-operating income or non-operating expense that may include amounts earned from dividends, interest on securities, gains (losses) on securities sold, equity earnings of unconsolidated affiliates, net gain (loss) on sales of business, interest expense and other miscellaneous income or expense items. + References No definition available. + Details
Name: | us-gaap_ScheduleOfOtherNonoperatingIncomeExpenseTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration