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Benefit Plans (Estimated Amoritization From Regulatory Assets and Regulatory Liabilities On The Consolidated Balance Sheets And Accumulated Other Comprehensive Loss On Alliant Energys Consolidated Balance Sheet) (Details) (Scenario, Forecast [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2013
Defined Benefit Pension Plans [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | $ 19.5
Prior service cost (credit) | 0
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | 19.5
Defined Benefit Pension Plans [Member] | IPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | 8.0
Prior service cost (credit) | 0
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | 8.0
Defined Benefit Pension Plans [Member] | WPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | 9.2
Prior service cost (credit) | 0.3
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | 9.5
Other Postretirement Benefits Plans [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | 2.4
Prior service cost (credit) | (11.9)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | (9.5)
Other Postretirement Benefits Plans [Member] | IPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | 1.1
Prior service cost (credit) | (6.3)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | (5.2)
Other Postretirement Benefits Plans [Member] | WPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | 1.2
Prior service cost (credit) | (3.9)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | $ (2.7)

X
- Definition Amount included in accumulated other comprehensive income (AOCI) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position, including gain (loss), prior service cost (credit), and transition asset (obligation). + References No definition available. + Details
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- Definition Amount included in accumulated other comprehensive income (AOCI) related to gain (loss) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position. + References No definition available. + Details
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