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Business Segments (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 USD ($) a | Sep. 30, 2015 USD ($) | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Sep. 30, 2014 USD ($) | Jun. 30, 2014 USD ($) | Mar. 31, 2014 USD ($) | Dec. 31, 2015 USD ($) a | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($)
Business Segments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments |  |  |  |  |  |  |  |  | During the year ended December 31, 2015, management updated our reportable segment presentation to reflect the manner in which we operate, assess, and allocate resources after our recent acquisitions. We now present three reportable segments, Regulated Operations, ALLETE Clean Energy, and U.S. Water Services. Prior period amounts have been revised to conform with the current business segment presentation. |  |
Number of Reportable Segments |  |  |  |  |  |  |  |  | 3 |  |
Operating Revenue | $ 380.6 | $ 462.5 | $ 323.3 | $ 320.0 | $ 290.7 | $ 288.9 | $ 260.7 | $ 296.5 | $ 1,486.4 | $ 1,136.8 | $ 1,018.4
Net Income (Loss) Attributable to Parent | 18.3 | $ 60.4 | $ 22.5 | $ 39.9 | 32.9 | $ 41.6 | $ 16.8 | $ 33.5 | 141.1 | 124.8 | 104.7
Depreciation and Amortization |  |  |  |  |  |  |  |  | 170.0 | 135.7 | 116.6
Impairment of Real Estate |  |  |  |  |  |  |  |  | 36.3 | 0.0 | 0.0
Interest Expense |  |  |  |  |  |  |  |  | 64.9 | 54.8 | 50.3
Equity Earnings in ATC |  |  |  |  |  |  |  |  | 16.3 | 19.6 | 20.3
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 25.3 | 36.7 | 28.7
Assets | 4,907.1 |  |  |  | 4,360.8 |  |  |  | 4,907.1 | 4,360.8 |
Capital Expenditures |  |  |  |  |  |  |  |  | $ 251.8 | 604.3 |
Regulated Operations [Member] |  |  |  |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of Operating Segments |  |  |  |  |  |  |  |  | 3 |  |
Operating Revenue |  |  |  |  |  |  |  |  | $ 991.2 | 1,003.5 | 925.5
Net Income (Loss) Attributable to Parent |  |  |  |  |  |  |  |  | 131.6 | 123.0 | 103.6
Depreciation and Amortization |  |  |  |  |  |  |  |  | 135.1 | 118.0 | 110.2
Interest Expense |  |  |  |  |  |  |  |  | 53.9 | 49.2 | 44.4
Equity Earnings in ATC |  |  |  |  |  |  |  |  | 16.3 | 19.6 | 20.3
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 24.4 | 39.0 | 35.1
Assets | 3,862.6 |  |  |  | 3,709.6 |  |  |  | 3,862.6 | 3,709.6 |
Capital Expenditures |  |  |  |  |  |  |  |  | 224.4 | 583.5 |
ALLETE Clean Energy [Member] |  |  |  |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating Revenue |  |  |  |  |  |  |  |  | 262.1 | 33.2 | 0.0
Net Income (Loss) Attributable to Parent |  |  |  |  |  |  |  |  | 29.9 | 3.3 | (3.4)
Depreciation and Amortization |  |  |  |  |  |  |  |  | 18.7 | 10.1 | 0.0
Interest Expense |  |  |  |  |  |  |  |  | 3.3 | 0.8 | 0.0
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 21.0 | 2.3 | (2.3)
Assets | 504.6 |  |  |  | 291.9 |  |  |  | 504.6 | 291.9 |
Capital Expenditures |  |  |  |  |  |  |  |  | 8.6 | 4.2 |
U.S. Water Services [Member] |  |  |  |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating Revenue |  |  |  |  |  |  |  |  | 119.8 | 0.0 | 0.0
Net Income (Loss) Attributable to Parent |  |  |  |  |  |  |  |  | 0.9 | 0.0 | 0.0
Depreciation and Amortization |  |  |  |  |  |  |  |  | 7.3 | 0.0 | 0.0
Interest Expense |  |  |  |  |  |  |  |  | 1.4 | 0.0 | 0.0
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 0.9 | 0.0 | 0.0
Assets | $ 258.3 |  |  |  | 0.0 |  |  |  | 258.3 | 0.0 |
Capital Expenditures |  |  |  |  |  |  |  |  | $ 2.9 | 0.0 |
Corporate and Other [Member] |  |  |  |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of Operating Segments |  |  |  |  |  |  |  |  | 2 |  |
Land in Minnesota (Acres) | a | 5,000 |  |  |  |  |  |  |  | 5,000 |  |
Operating Revenue |  |  |  |  |  |  |  |  | $ 113.3 | 100.1 | 92.9
Net Income (Loss) Attributable to Parent |  |  |  |  |  |  |  |  | (21.3) | (1.5) | 4.5
Depreciation and Amortization |  |  |  |  |  |  |  |  | 8.9 | 7.6 | 6.4
Impairment of Real Estate |  |  |  |  |  |  |  |  | 36.3 | 0.0 | 0.0
Interest Expense |  |  |  |  |  |  |  |  | 8.6 | 7.1 | 8.2
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | (21.0) | (4.6) | (4.1)
Assets | $ 281.6 |  |  |  | $ 359.3 |  |  |  | 281.6 | 359.3 |
Capital Expenditures |  |  |  |  |  |  |  |  | 15.9 | 16.6 |
Eliminations [Member] |  |  |  |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest Expense |  |  |  |  |  |  |  |  | $ (2.3) | $ (2.3) | $ (2.3)

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