Benefit Plans (Funded Status of Benefits Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	$ 1,279.7	$ 1,175.0
Change in benefit obligation:
Net benefit obligation at January 1	1,175.0	1,303.1
Service cost	9.7	12.1	$ 12.5
Interest cost	49.8	46.8	51.0
Plan participants' contributions	0.0	0.0
Actuarial (gain) loss	124.6	(96.2)
Gross benefits paid	(79.4)	(90.8)
Net benefit obligation at December 31	1,279.7	1,175.0	1,303.1
Change in plan assets:
Fair value of plan assets at January 1	808.6	950.7
Actual return on plan assets	168.1	(57.8)
Employer contributions	33.1	6.5
Plan participants' contributions	0.0	0.0
Gross benefits paid	(79.4)	(90.8)
Fair value of plan assets at December 31	930.4	808.6	950.7
Under funded status at December 31	(349.3)	(366.4)
Fair value of plan assets	930.4	808.6
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	581.2	533.9
Change in benefit obligation:
Net benefit obligation at January 1	533.9	592.9
Service cost	6.0	7.4	7.3
Interest cost	22.7	21.4	23.5
Plan participants' contributions	0.0	0.0
Actuarial (gain) loss	55.9	(44.4)
Gross benefits paid	(37.3)	(43.4)
Net benefit obligation at December 31	581.2	533.9	592.9
Change in plan assets:
Fair value of plan assets at January 1	377.8	443.7
Actual return on plan assets	78.9	(26.8)
Employer contributions	16.6	4.3
Plan participants' contributions	0.0	0.0
Gross benefits paid	(37.3)	(43.4)
Fair value of plan assets at December 31	436.0	377.8	443.7
Under funded status at December 31	(145.2)	(156.1)
Fair value of plan assets	436.0	377.8
Pension Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	550.1	506.6
Change in benefit obligation:
Net benefit obligation at January 1	506.6	559.8
Service cost	3.5	4.4	4.9
Interest cost	21.5	20.2	21.8
Plan participants' contributions	0.0	0.0
Actuarial (gain) loss	53.8	(40.0)
Gross benefits paid	(35.3)	(37.8)
Net benefit obligation at December 31	550.1	506.6	559.8
Change in plan assets:
Fair value of plan assets at January 1	352.2	415.0
Actual return on plan assets	73.4	(25.1)
Employer contributions	15.7	0.1
Plan participants' contributions	0.0	0.0
Gross benefits paid	(35.3)	(37.8)
Fair value of plan assets at December 31	406.0	352.2	415.0
Under funded status at December 31	(144.1)	(154.4)
Fair value of plan assets	406.0	352.2
Other Postretirement Benefits Plans [Member]
Change in benefit obligation:
Net benefit obligation at January 1	206.1	222.3
Service cost	3.3	4.2	5.0
Interest cost	8.5	7.7	8.6
Plan participants' contributions	3.4	3.1
Actuarial (gain) loss	13.6	(8.2)
Gross benefits paid	(21.3)	(23.0)
Net benefit obligation at December 31	213.6	206.1	222.3
Change in plan assets:
Fair value of plan assets at January 1	99.1	111.1
Actual return on plan assets	14.6	(2.6)
Employer contributions	9.1	10.5
Plan participants' contributions	3.4	3.1
Gross benefits paid	(21.3)	(23.0)
Fair value of plan assets at December 31	104.9	99.1	111.1
Under funded status at December 31	(108.7)	(107.0)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	82.5	89.4
Service cost	1.3	1.7	2.1
Interest cost	3.4	3.1	3.5
Plan participants' contributions	1.3	1.0
Actuarial (gain) loss	5.7	(4.3)
Gross benefits paid	(8.5)	(8.4)
Net benefit obligation at December 31	85.7	82.5	89.4
Change in plan assets:
Fair value of plan assets at January 1	66.7	72.9
Actual return on plan assets	10.1	(1.4)
Employer contributions	2.3	2.6
Plan participants' contributions	1.3	1.0
Gross benefits paid	(8.5)	(8.4)
Fair value of plan assets at December 31	71.9	66.7	72.9
Under funded status at December 31	(13.8)	(15.8)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	83.2	90.4
Service cost	1.3	1.6	1.9
Interest cost	3.3	3.1	3.4
Plan participants' contributions	1.5	1.4
Actuarial (gain) loss	5.0	(2.5)
Gross benefits paid	(9.0)	(10.8)
Net benefit obligation at December 31	85.3	83.2	90.4
Change in plan assets:
Fair value of plan assets at January 1	16.7	18.7
Actual return on plan assets	1.7	(0.1)
Employer contributions	6.2	7.5
Plan participants' contributions	1.5	1.4
Gross benefits paid	(9.0)	(10.8)
Fair value of plan assets at December 31	17.1	16.7	$ 18.7
Under funded status at December 31	$ (68.2)	$ (66.5)