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Benefit Plans (Fair Values Of Nonvested Performance Shares And Units) (Details) - $ / shares | 6 Months Ended |  |  |
--- | --- | --- | --- | ---
Jun. 30, 2015 | Dec. 31, 2014 | Jun. 30, 2014 | Dec. 31, 2013
Performance Shares [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Nonvested awards (in shares/awards) | [1] | 144,215 | 144,424 | 144,424 | 139,940
Performance Units [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Nonvested awards (in shares/awards) | [2] | 58,324 | 63,665 | 64,822 | 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 June 30, 2015 (in dollars per share) |  | $ 57.72 |  |  |
Estimated payout percentage based on performance criteria |  | 100.00% |  |  |
Fair values of each nonvested award (in dollars per share) |  | $ 57.72 |  |  |
2015 Grant [Member] | Performance Units [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Nonvested awards (in shares/awards) |  | 17,504 |  |  |
Alliant Energy common stock closing price on grant date (in dollars per share) |  | $ 65.09 |  |  |
Estimated payout percentage based on performance criteria |  | 100.00% |  |  |
Fair values of each nonvested award (in dollars per share) |  | $ 65.09 |  |  |
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 June 30, 2015 (in dollars per share) |  | $ 57.72 |  |  |
Estimated payout percentage based on performance criteria |  | 133.00% |  |  |
Fair values of each nonvested award (in dollars per share) |  | $ 76.77 |  |  |
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 |  | 133.00% |  |  |
Fair values of each nonvested award (in dollars per share) |  | $ 71.51 |  |  |
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 June 30, 2015 (in dollars per share) |  | $ 57.72 |  |  |
Estimated payout percentage based on performance criteria |  | 180.00% |  |  |
Fair values of each nonvested award (in dollars per share) |  | $ 103.90 |  |  |
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 |  | 180.00% |  |  |
Fair values of each nonvested award (in dollars per share) |  | $ 85.64 |  |  |
[1] | Share amounts represent the target number of performance shares.
[2] | Unit amounts represent the target number of performance units.

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