Benefit Plans (Funded Status of Benefits Plans) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
IPL [Member]
Change in benefit obligation:
Regulatory assets	$ 38.0	$ 38.2
WPL [Member]
Change in benefit obligation:
Regulatory assets	29.5	28.0
Pension Plans, Defined Benefit [Member]
Change in benefit obligation:
Net benefit obligation at January 1	1,301.5	1,113.4
Service cost	15.9	13.1	$ 15.7
Interest cost	53.6	54.1	49.0
Plan participants' contributions	0.0	0.0
Plan amendments	0.0	0.0
Additional benefit costs	0.5	0.0
Actuarial (gain) loss	(70.1)	195.8
Gross benefits paid	(95.1)	(74.9)
Net benefit obligation at December 31	1,206.3	1,301.5	1,113.4
Change in plan assets:
Fair value of plan assets at January 1	1,018.1	1,022.9
Actual return on plan assets	(30.2)	66.4
Employer contributions	2.2	3.7
Plan participants' contributions	0.0	0.0
Gross benefits paid	(95.1)	(74.9)
Fair value of plan assets at December 31	895.0	1,018.1	1,022.9
Under funded status at December 31	(311.3)	(283.4)
Pension Plans, Defined Benefit [Member] \| IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	603.1	514.0
Service cost	8.8	7.2	8.6
Interest cost	25.0	25.1	22.9
Plan participants' contributions	0.0	0.0
Plan amendments	0.0	0.0
Actuarial (gain) loss	(32.3)	91.4
Gross benefits paid	(48.5)	(34.6)
Net benefit obligation at December 31	556.1	603.1	514.0
Change in plan assets:
Fair value of plan assets at January 1	484.7	485.9
Actual return on plan assets	(14.3)	32.1
Employer contributions	0.8	1.3
Plan participants' contributions	0.0	0.0
Gross benefits paid	(48.5)	(34.6)
Fair value of plan assets at December 31	422.7	484.7	485.9
Under funded status at December 31	(133.4)	(118.4)
Pension Plans, Defined Benefit [Member] \| WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	547.6	460.8
Service cost	5.8	4.9	5.9
Interest cost	22.6	22.6	20.7
Plan participants' contributions	0.0	0.0
Plan amendments	0.0	0.0
Additional benefit costs	0.5	0.0
Actuarial (gain) loss	(30.0)	86.7
Gross benefits paid	(40.6)	(27.4)
Net benefit obligation at December 31	505.9	547.6	460.8
Change in plan assets:
Fair value of plan assets at January 1	440.3	438.8
Actual return on plan assets	(13.0)	28.6
Employer contributions	0.1	0.3
Plan participants' contributions	0.0	0.0
Gross benefits paid	(40.6)	(27.4)
Fair value of plan assets at December 31	386.8	440.3	438.8
Under funded status at December 31	(119.1)	(107.3)
Other Postretirement Benefits Plans [Member]
Change in benefit obligation:
Net benefit obligation at January 1	231.1	208.7
Service cost	5.5	5.2	6.3
Interest cost	9.1	9.5	8.5
Plan participants' contributions	3.1	2.8
Plan amendments	(0.3)	0.0
Additional benefit costs	0.0	0.0
Actuarial (gain) loss	(9.4)	22.3
Gross benefits paid	(17.7)	(17.4)
Net benefit obligation at December 31	221.4	231.1	208.7
Change in plan assets:
Fair value of plan assets at January 1	121.6	124.9
Actual return on plan assets	(4.9)	5.6
Employer contributions	4.8	5.7
Plan participants' contributions	3.1	2.8
Gross benefits paid	(17.7)	(17.4)
Fair value of plan assets at December 31	106.9	121.6	124.9
Under funded status at December 31	(114.5)	(109.5)
Other Postretirement Benefits Plans [Member] \| IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	96.4	87.8
Service cost	2.4	2.4	2.9
Interest cost	3.8	3.9	3.6
Plan participants' contributions	1.0	0.9
Plan amendments	(0.1)	0.0
Actuarial (gain) loss	(4.6)	8.6
Gross benefits paid	(7.6)	(7.2)
Net benefit obligation at December 31	91.3	96.4	87.8
Change in plan assets:
Fair value of plan assets at January 1	78.7	81.2
Actual return on plan assets	(3.1)	3.6
Employer contributions	0.2	0.2
Plan participants' contributions	1.0	0.9
Gross benefits paid	(7.6)	(7.2)
Fair value of plan assets at December 31	69.2	78.7	81.2
Under funded status at December 31	(22.1)	(17.7)
Other Postretirement Benefits Plans [Member] \| WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	94.0	85.6
Service cost	2.1	2.0	2.5
Interest cost	3.7	3.8	3.4
Plan participants' contributions	1.6	1.3
Plan amendments	(0.2)	0.0
Additional benefit costs	0.0	0.0
Actuarial (gain) loss	(3.5)	9.2
Gross benefits paid	(8.0)	(7.9)
Net benefit obligation at December 31	89.7	94.0	85.6
Change in plan assets:
Fair value of plan assets at January 1	21.8	21.7
Actual return on plan assets	(1.1)	1.2
Employer contributions	4.4	5.5
Plan participants' contributions	1.6	1.3
Gross benefits paid	(8.0)	(7.9)
Fair value of plan assets at December 31	18.7	21.8	$ 21.7
Under funded status at December 31	$ (71.0)	$ (72.2)

Defined Benefit Plan, Additional Benefit Costs

No definition available.

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- Definition

Defined Benefit Plan Regulatory Asset Allocated From Corporate Services

+ References

No definition available.

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- Definition

The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Actual Return on Plan Assets (Component of Net Periodic Postretirement Benefit Cost)
-URI http://asc.fasb.org/extlink&oid=6504192

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 50
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-Subparagraph (b)(1)
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-Publisher FASB
-Name Accounting Standards Codification
-Glossary Actual Return on Plan Assets (Component of Net Periodic Pension Cost)
-URI http://asc.fasb.org/extlink&oid=6504226

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- Definition

Amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover, salary, and temporary deviation from the substantive plan.

+ References

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- Definition

1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Projected Benefit Obligation
-URI http://asc.fasb.org/extlink&oid=6522206

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Accumulated Postretirement Benefit Obligation
-URI http://asc.fasb.org/extlink&oid=6503904

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- Definition

The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

+ References

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

The increase in the fair value of plan assets from contributions made by the employer.

+ References

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- Definition

The amount of contributions made by plan participants. This item represents a periodic increase to the plan obligation and an increase to plan assets.

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Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

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- Definition

The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

+ References

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-Publisher FASB
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- Definition

The amount of increase or decrease due to a change in the terms of an existing plan or the initiation of a new plan. A plan amendment may increase or decrease benefits, including those attributed to years of service already rendered.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Service Cost (Component of Net Periodic Pension Cost)
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