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Benefit Plans (Fair Values Of Nonvested Performance Shares And Units) (Details) (USD $) |  |  |  |  |  |  |  |  | 3 Months Ended
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Mar. 31, 2013 Performance Shares [Member] | Dec. 31, 2012 Performance Shares [Member] | Mar. 31, 2012 Performance Shares [Member] | Dec. 31, 2011 Performance Shares [Member] | Mar. 31, 2013 Performance Units [Member] | Dec. 31, 2012 Performance Units [Member] | Mar. 31, 2012 Performance Units [Member] | Dec. 31, 2011 Performance Units [Member] | Mar. 31, 2013 2013 Grant [Member] Performance Shares [Member] | Mar. 31, 2013 2013 Grant [Member] Performance Units [Member] | Mar. 31, 2013 2012 Grant [Member] Performance Shares [Member] | Mar. 31, 2013 2012 Grant [Member] Performance Units [Member] | Mar. 31, 2013 2011 Grant [Member] Performance Shares [Member] | Mar. 31, 2013 2011 Grant [Member] Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Nonvested awards (in shares) | 139,940 | [1] | 145,277 | [1] | 170,611 | [1] | 236,979 | [1] | 66,397 | [2] | 64,969 | [2] | 66,804 | [2] | 42,996 | [2] | 49,093 | 22,201 | 45,612 | 23,392 | 45,235 | 20,804
Alliant Energy common stock closing price on March 31, 2013 (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 50.18 |  | $ 50.18 |  | $ 50.18 |
Alliant Energy common stock closing price on grant date (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 47.58 |  | $ 43.05 |  | $ 38.75
Estimated payout percentage based on performance criteria |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 97.00% | 97.00% | 92.00% | 92.00% | 112.00% | 112.00%
Fair values of each nonvested award (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 48.67 | $ 46.15 | $ 46.17 | $ 39.61 | $ 56.20 | $ 43.40
[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] | 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.

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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(b) -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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(a) -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. + Details
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