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Benefit Plans (Fair Values Of Nonvested Performance Shares And Units) (Details) - $ / shares | 9 Months Ended |  |  |
--- | --- | --- | --- | ---
Sep. 30, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Dec. 31, 2013
Performance Shares [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Nonvested awards (in shares/awards) | 144,215 | 144,424 | 144,424 | 139,940
Performance Units [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Nonvested awards (in shares/awards) | 58,206 | 63,665 | 64,625 | 65,912
2015 Grant [Member] | Performance Shares [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Nonvested awards (in shares/awards) | 45,403 |  |  |
Alliant Energy common stock closing price on September 30, 2015 (in dollars per share) | $ 58.49 |  |  |
Estimated payout percentage based on performance criteria | 70.00% |  |  |
Fair values of each nonvested award (in dollars per share) | $ 40.94 |  |  |
2015 Grant [Member] | Performance Units [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Nonvested awards (in shares/awards) | 17,386 |  |  |
Alliant Energy common stock closing price on grant date (in dollars per share) | $ 65.09 |  |  |
Estimated payout percentage based on performance criteria | 70.00% |  |  |
Fair values of each nonvested award (in dollars per share) | $ 45.56 |  |  |
2014 Grant [Member] | Performance Shares [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Nonvested awards (in shares/awards) | 49,719 |  |  |
Alliant Energy common stock closing price on September 30, 2015 (in dollars per share) | $ 58.49 |  |  |
Estimated payout percentage based on performance criteria | 105.00% |  |  |
Fair values of each nonvested award (in dollars per share) | $ 61.41 |  |  |
2014 Grant [Member] | Performance Units [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Nonvested awards (in shares/awards) | 19,440 |  |  |
Alliant Energy common stock closing price on grant date (in dollars per share) | $ 53.77 |  |  |
Estimated payout percentage based on performance criteria | 105.00% |  |  |
Fair values of each nonvested award (in dollars per share) | $ 56.46 |  |  |
2013 Grant [Member] | Performance Shares [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Nonvested awards (in shares/awards) | 49,093 |  |  |
Alliant Energy common stock closing price on September 30, 2015 (in dollars per share) | $ 58.49 |  |  |
Estimated payout percentage based on performance criteria | 150.00% |  |  |
Fair values of each nonvested award (in dollars per share) | $ 87.74 |  |  |
2013 Grant [Member] | Performance Units [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Nonvested awards (in shares/awards) | 21,380 |  |  |
Alliant Energy common stock closing price on grant date (in dollars per share) | $ 47.58 |  |  |
Estimated payout percentage based on performance criteria | 150.00% |  |  |
Fair values of each nonvested award (in dollars per share) | $ 71.37 |  |  |

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