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Benefit Plans (Summary Of Performance Unit Activity) (Details) (Performance Units [Member], USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 6 Months Ended
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Mar. 31, 2013 | Jun. 30, 2013 | Jun. 30, 2012
Performance Units [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward] |  |  |  |  |  |
Nonvested, January 1 (in shares/awards) | 64,969 | [1] | 64,969 | [1] | 42,996 | [1]
Granted (in shares) |  |  | 22,201 | [1] | 24,686 | [1]
Vested (in shares) | (19,760) |  | (19,760) | [1],[2] | 0 | [1],[2]
Forfeited (in shares) |  |  | (1,013) | [1] | (878) | [1]
Nonvested, June 30 (in shares/awards) |  |  | 66,397 | [1] | 66,804 | [1]
Performance multiplier range, lower |  |  | 0.00% |  |  |
Performance multiplier range, upper |  |  | 200.00% |  |  |
Vested percentage of the target | 197.50% |  |  |  |  |
Stock payout value | $ 1.3 |  |  |  |  |
[1] | Unit amounts represent the target number of performance units. Each performance unit’s value is based on the closing price of one share of Alliant Energy’s common stock on the grant date of the award. The actual payout for performance units is dependent upon actual performance and may range from zero to 200% of the target number of units.
[2] | In the first quarter of 2013, 19,760 performance units granted in 2010 vested at 197.5% of the target, resulting in cash payouts valued at $1.3 million.

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- Definition Costs and payments related to employee benefits and equity-based compensation, such as pension expense and contributions, other postretirement benefits expense and payments, stock or unit options expense, and amortization of restricted stock or unit. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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