Benefit Plans (Funded Status of Benefits Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plans, Defined Benefit [Member]
Change in benefit obligation:
Net benefit obligation at January 1	$ 1,206.3	$ 1,301.5
Service cost	12.6	15.9	$ 13.1
Interest cost	53.0	53.6	54.1
Plan participants' contributions	0.0	0.0
Plan amendments	0.0	0.0
Additional benefit costs	0.0	0.5
Actuarial (gain) loss	48.3	(70.1)
Gross benefits paid	(75.9)	(95.1)
Net benefit obligation at December 31	1,244.3	1,206.3	1,301.5
Change in plan assets:
Fair value of plan assets at January 1	895.0	1,018.1
Actual return on plan assets	74.3	(30.2)
Employer contributions	2.3	2.2
Plan participants' contributions	0.0	0.0
Gross benefits paid	(75.9)	(95.1)
Fair value of plan assets at December 31	895.7	895.0	1,018.1
Under funded status at December 31	(348.6)	(311.3)
Pension Plans, Defined Benefit [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	556.1	603.1
Service cost	7.5	8.8	7.2
Interest cost	24.5	25.0	25.1
Plan participants' contributions	0.0	0.0
Plan amendments	0.0	0.0
Actuarial (gain) loss	19.1	(32.3)
Gross benefits paid	(36.8)	(48.5)
Net benefit obligation at December 31	570.4	556.1	603.1
Change in plan assets:
Fair value of plan assets at January 1	422.7	484.7
Actual return on plan assets	35.3	(14.3)
Employer contributions	0.8	0.8
Plan participants' contributions	0.0	0.0
Gross benefits paid	(36.8)	(48.5)
Fair value of plan assets at December 31	422.0	422.7	484.7
Under funded status at December 31	(148.4)	(133.4)
Pension Plans, Defined Benefit [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	505.9	547.6
Service cost	4.9	5.8	4.9
Interest cost	22.3	22.6	22.6
Plan participants' contributions	0.0	0.0
Plan amendments	0.0	0.0
Additional benefit costs	0.0	0.5
Actuarial (gain) loss	25.7	(30.0)
Gross benefits paid	(29.6)	(40.6)
Net benefit obligation at December 31	529.2	505.9	547.6
Change in plan assets:
Fair value of plan assets at January 1	386.8	440.3
Actual return on plan assets	32.4	(13.0)
Employer contributions	0.1	0.1
Plan participants' contributions	0.0	0.0
Gross benefits paid	(29.6)	(40.6)
Fair value of plan assets at December 31	389.7	386.8	440.3
Under funded status at December 31	(139.5)	(119.1)
Other Postretirement Benefits Plans [Member]
Change in benefit obligation:
Net benefit obligation at January 1	221.4	231.1
Service cost	5.3	5.5	5.2
Interest cost	9.4	9.1	9.5
Plan participants' contributions	2.4	3.1
Plan amendments	0.0	(0.3)
Additional benefit costs	0.0	0.0
Actuarial (gain) loss	(0.3)	(9.4)
Gross benefits paid	(18.1)	(17.7)
Net benefit obligation at December 31	220.1	221.4	231.1
Change in plan assets:
Fair value of plan assets at January 1	106.9	121.6
Actual return on plan assets	8.2	(4.9)
Employer contributions	6.4	4.8
Plan participants' contributions	2.4	3.1
Gross benefits paid	(18.1)	(17.7)
Fair value of plan assets at December 31	105.8	106.9	121.6
Under funded status at December 31	(114.3)	(114.5)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	91.3	96.4
Service cost	2.3	2.4	2.4
Interest cost	3.8	3.8	3.9
Plan participants' contributions	0.9	1.0
Plan amendments	0.0	(0.1)
Actuarial (gain) loss	(0.7)	(4.6)
Gross benefits paid	(7.5)	(7.6)
Net benefit obligation at December 31	90.1	91.3	96.4
Change in plan assets:
Fair value of plan assets at January 1	69.2	78.7
Actual return on plan assets	5.3	(3.1)
Employer contributions	0.3	0.2
Plan participants' contributions	0.9	1.0
Gross benefits paid	(7.5)	(7.6)
Fair value of plan assets at December 31	68.2	69.2	78.7
Under funded status at December 31	(21.9)	(22.1)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	89.7	94.0
Service cost	2.0	2.1	2.0
Interest cost	3.8	3.7	3.8
Plan participants' contributions	1.2	1.6
Plan amendments	0.0	(0.2)
Additional benefit costs	0.0	0.0
Actuarial (gain) loss	0.5	(3.5)
Gross benefits paid	(8.3)	(8.0)
Net benefit obligation at December 31	88.9	89.7	94.0
Change in plan assets:
Fair value of plan assets at January 1	18.7	21.8
Actual return on plan assets	1.2	(1.1)
Employer contributions	5.8	4.4
Plan participants' contributions	1.2	1.6
Gross benefits paid	(8.3)	(8.0)
Fair value of plan assets at December 31	18.6	18.7	$ 21.8
Under funded status at December 31	$ (70.3)	$ (71.0)