Benefit Plans (Funded Status of Benefits Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	$ 1,351	$ 1,280
Change in benefit obligation:
Net benefit obligation at January 1	1,280	1,175
Service cost	11	10	$ 12
Interest cost	43	50	47
Plan participants' contributions	0	0
Actuarial (gain) loss	132	124
Gross benefits paid	(115)	(79)
Net benefit obligation at December 31	1,351	1,280	1,175
Change in plan assets:
Fair value of plan assets at January 1	931	809
Actual return on plan assets	108	168
Employer contributions	60	33
Plan participants' contributions	0	0
Gross benefits paid	(115)	(79)
Fair value of plan assets at December 31	984	931	809
Under funded status at December 31	(367)	(349)
Fair value of plan assets	984	931
Pension Plans, Defined Benefit [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	615	581
Change in benefit obligation:
Net benefit obligation at January 1	581	534
Service cost	7	6	7
Interest cost	20	23	21
Plan participants' contributions	0	0
Actuarial (gain) loss	60	56
Gross benefits paid	(53)	(38)
Net benefit obligation at December 31	615	581	534
Change in plan assets:
Fair value of plan assets at January 1	436	378
Actual return on plan assets	51	79
Employer contributions	19	17
Plan participants' contributions	0	0
Gross benefits paid	(53)	(38)
Fair value of plan assets at December 31	453	436	378
Under funded status at December 31	(162)	(145)
Fair value of plan assets	453	436
Pension Plans, Defined Benefit [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	588	550
Change in benefit obligation:
Net benefit obligation at January 1	550	507
Service cost	4	3	4
Interest cost	19	21	20
Plan participants' contributions	0	0
Actuarial (gain) loss	55	54
Gross benefits paid	(40)	(35)
Net benefit obligation at December 31	588	550	507
Change in plan assets:
Fair value of plan assets at January 1	406	352
Actual return on plan assets	48	73
Employer contributions	22	16
Plan participants' contributions	0	0
Gross benefits paid	(40)	(35)
Fair value of plan assets at December 31	436	406	352
Under funded status at December 31	(152)	(144)
Fair value of plan assets	436	406
Other Postretirement Benefits Plans [Member]
Change in benefit obligation:
Net benefit obligation at January 1	214	206
Service cost	3	3	4
Interest cost	7	9	8
Plan participants' contributions	4	3
Actuarial (gain) loss	18	14
Gross benefits paid	(19)	(21)
Net benefit obligation at December 31	227	214	206
Change in plan assets:
Fair value of plan assets at January 1	105	99
Actual return on plan assets	11	15
Employer contributions	7	9
Plan participants' contributions	4	3
Gross benefits paid	(19)	(21)
Fair value of plan assets at December 31	108	105	99
Under funded status at December 31	(119)	(109)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	86	83
Service cost	1	1	2
Interest cost	3	3	3
Plan participants' contributions	1	1
Actuarial (gain) loss	8	6
Gross benefits paid	(8)	(8)
Net benefit obligation at December 31	91	86	83
Change in plan assets:
Fair value of plan assets at January 1	72	67
Actual return on plan assets	7	10
Employer contributions	2	2
Plan participants' contributions	1	1
Gross benefits paid	(8)	(8)
Fair value of plan assets at December 31	74	72	67
Under funded status at December 31	(17)	(14)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	85	83
Service cost	2	1	2
Interest cost	3	3	3
Plan participants' contributions	2	2
Actuarial (gain) loss	5	5
Gross benefits paid	(8)	(9)
Net benefit obligation at December 31	89	85	83
Change in plan assets:
Fair value of plan assets at January 1	17	17
Actual return on plan assets	2	1
Employer contributions	5	6
Plan participants' contributions	2	2
Gross benefits paid	(8)	(9)
Fair value of plan assets at December 31	18	17	$ 17
Under funded status at December 31	$ (71)	$ (68)