Benefit Plans (Fair Values Of Nonvested Performance Shares And Units) (Details) (USD $)

3 Months Ended

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

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%

92.00%

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.

- 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+ Details

INTERSTATE POWER & LIGHT CO 10-Q