Benefit Plans (Fair Values Of Nonvested Performance Shares And Performance Units) (Details)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	223,511
Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	71,737
2017 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	65,350
Alliant Energy common stock closing price on December 29, 2017 (in dollars per share)	$ 42.61
Estimated payout percentage based on performance criteria	105.00%
Fair values of each nonvested award (in dollars per share)	$ 44.74
2017 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	18,600
Alliant Energy common stock closing price on December 29, 2017 (in dollars per share)	$ 42.61
Estimated payout percentage based on performance criteria	105.00%
Fair values of each nonvested award (in dollars per share)	$ 44.74
2016 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	67,355
Alliant Energy common stock closing price on December 29, 2017 (in dollars per share)	$ 42.61
Estimated payout percentage based on performance criteria	150.00%
Fair values of each nonvested award (in dollars per share)	$ 63.92
2016 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	21,227
Alliant Energy common stock closing price on December 29, 2017 (in dollars per share)	$ 42.61
Estimated payout percentage based on performance criteria	150.00%
Fair values of each nonvested award (in dollars per share)	$ 63.92
2015 Grant [Member] | Performance Shares [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	90,806
Alliant Energy common stock closing price on December 29, 2017 (in dollars per share)	$ 42.61
Estimated payout percentage based on performance criteria	138.00%
Fair values of each nonvested award (in dollars per share)	$ 58.80
2015 Grant [Member] | Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested, December 31 (in shares/awards) | shares	31,910
Alliant Energy common stock closing price on grant date (in dollars per share)	$ 32.55
Estimated payout percentage based on performance criteria	138.00%
Fair values of each nonvested award (in dollars per share)	$ 44.92