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Benefit Plans (Summary Of Performance Shares Activity) (Details) (Performance Shares [Member], USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Mar. 31, 2013 | Mar. 31, 2012 | Jun. 30, 2013 | Jun. 30, 2012
Performance Shares [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward] |  |  |  |  |  |  |  |
Nonvested, January 1 (in shares/awards) | 145,277 | [1] | 236,979 | [1] | 145,277 | [1] | 236,979 | [1]
Granted (in shares) |  |  |  |  | 49,093 | [1] | 45,612 | [1]
Vested (in shares) | (54,430) |  | (111,980) |  | (54,430) | [1],[2] | (111,980) | [1],[2]
Forfeited (in shares) |  |  |  |  | 0 | [1] | (25,334) | [1]
Nonvested, June 30 (in shares/awards) |  |  |  |  | 139,940 | [1] | 145,277 | [1]
Actual number of shares paid out upon vesting, minimum percentage of target shares |  |  |  |  | 0.00% |  |  |
Actual number of shares paid out upon vesting, maximum percentage of target shares |  |  |  |  | 200.00% |  |  |
Stock payout value | $ 4.8 |  | $ 8.0 |  |  |  |  |
Vested percentage of the target | 197.50% |  | 162.50% |  |  |  |  |
Common stock shares from vested performance shares (in shares) | 4,177 |  | 6,399 |  |  |  |  |
[1] | Share amounts represent the target number of performance shares. Each performance share’s value is based on the price of one share of Alliant Energy’s common stock at the end of the performance period. The actual number of shares that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of shares.
[2] | In the first quarter of 2013, 54,430 performance shares granted in 2010 vested at 197.5% of the target, resulting in payouts valued at $4.8 million, which consisted of a combination of cash and common stock (4,177 shares). In the first quarter of 2012, 111,980 performance shares granted in 2009 vested at 162.5% of the target, resulting in payouts valued at $8.0 million, which consisted of a combination of cash and common stock (6,399 shares).

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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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