Benefit Plans (Fair Values Of Nonvested Performance Shares And Units) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014		Dec. 31, 2013
Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards (in shares/awards)	144,215	[1]	144,424	[1]	145,926	[1]	139,940	[1]
Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards (in shares/awards)	58,564	[2]	63,665	[2]	65,272	[2]	65,912	[2]
2015 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards (in shares/awards)	45,403
Alliant Energy common stock closing price on March 31, 2015 (in dollars per share)	$ 63.00
Estimated payout percentage based on performance criteria	100.00%
Fair values of each nonvested award (in dollars per share)	$ 63.00
2015 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards (in shares/awards)	17,744
Alliant Energy common stock closing price on grant date (in dollars per share)	$ 65.09
Estimated payout percentage based on performance criteria	100.00%
Fair values of each nonvested award (in dollars per share)	$ 65.09
2014 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards (in shares/awards)	49,719
Alliant Energy common stock closing price on March 31, 2015 (in dollars per share)	$ 63.00
Estimated payout percentage based on performance criteria	128.00%
Fair values of each nonvested award (in dollars per share)	$ 80.64
2014 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards (in shares/awards)	19,440
Alliant Energy common stock closing price on grant date (in dollars per share)	$ 53.77
Estimated payout percentage based on performance criteria	128.00%
Fair values of each nonvested award (in dollars per share)	$ 68.83
2013 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards (in shares/awards)	49,093
Alliant Energy common stock closing price on March 31, 2015 (in dollars per share)	$ 63.00
Estimated payout percentage based on performance criteria	178.00%
Fair values of each nonvested award (in dollars per share)	$ 112.14
2013 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards (in shares/awards)	21,380
Alliant Energy common stock closing price on grant date (in dollars per share)	$ 47.58
Estimated payout percentage based on performance criteria	178.00%
Fair values of each nonvested award (in dollars per share)	$ 84.69

[1] Share amounts represent the target number of performance shares.

[2] Unit amounts represent the target number of performance units.