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Benefit Plans (Summary Of Performance Shares Activity) (Details) (Performance Shares [Member], USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
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] | 234,518 | [1]
Granted (in shares) | 49,093 | [1] | 45,612 | [1] | 64,217 | [1]
Vested (in shares) | (54,430) | [1],[2] | (111,980) | [1],[2] | (57,838) | [1],[2]
Forfeited (in shares) | 0 | [1],[3] | (25,334) | [1],[3] | (3,918) | [1],[3]
Nonvested, December 31 (in shares/awards) | 139,940 | [1] | 145,277 | [1] | 236,979 | [1]
Instrument valuation based on shares of common stock, number of shares | 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% |  |  |  |  |
Cash and stock payout value | $ 4.8 |  | $ 8.0 |  | $ 1.6 |
Vested percentage of the target | 197.50% |  | 162.50% |  | 75.00% |
Common stock shares from vested performance shares (in shares) | 4,177 |  | 6,399 |  | 1,387 |
[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 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 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). In 2011, 57,838 performance shares granted in 2008 vested at 75% of the target, resulting in payouts valued at $1.6 million, which consisted of a combination of cash and common stock (1,387 shares).
[3] | Forfeitures were primarily caused by retirements and voluntary terminations of participants.

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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 Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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