Benefit Plans (Fair Values Of Nonvested Performance Shares And Units) (Details) - $ / shares	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards (in shares/awards)	144,215	144,424	144,424	139,940
Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards (in shares/awards)	58,206	63,665	64,625	65,912
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 September 30, 2015 (in dollars per share)	$ 58.49
Estimated payout percentage based on performance criteria	70.00%
Fair values of each nonvested award (in dollars per share)	$ 40.94
2015 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards (in shares/awards)	17,386
Alliant Energy common stock closing price on grant date (in dollars per share)	$ 65.09
Estimated payout percentage based on performance criteria	70.00%
Fair values of each nonvested award (in dollars per share)	$ 45.56
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 September 30, 2015 (in dollars per share)	$ 58.49
Estimated payout percentage based on performance criteria	105.00%
Fair values of each nonvested award (in dollars per share)	$ 61.41
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	105.00%
Fair values of each nonvested award (in dollars per share)	$ 56.46
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 September 30, 2015 (in dollars per share)	$ 58.49
Estimated payout percentage based on performance criteria	150.00%
Fair values of each nonvested award (in dollars per share)	$ 87.74
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	150.00%
Fair values of each nonvested award (in dollars per share)	$ 71.37