Benefit Plans (Funded Status of Benefits Plans) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plans, Defined Benefit [Member]
Change in benefit obligation:
Net benefit obligation at January 1	$ 1,113.4	$ 1,207.5
Service cost	13.1	15.7	13.5
Interest cost	54.1	49.0	51.6
Plan participants' contributions	0	0
Additional benefit costs	0	9.0
Actuarial (gain) loss	195.8	(94.1)
Gross benefits paid	(74.9)	(73.7)
Net benefit obligation at December 31	1,301.5	1,113.4	1,207.5
Change in plan assets:
Fair value of plan assets at January 1	1,022.9	965.6
Actual return on plan assets	66.4	128.5
Employer contributions	3.7	2.5
Plan participants' contributions	0	0
Gross benefits paid	(74.9)	(73.7)
Fair value of plan assets at December 31	1,018.1	1,022.9	965.6
Under funded status at December 31	(283.4)	(90.5)
Pension Plans, Defined Benefit [Member] \| IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	514.0	559.2
Service cost	7.2	8.6	7.5
Interest cost	25.1	22.9	24.1
Plan participants' contributions	0	0
Additional benefit costs	0	2.6
Actuarial (gain) loss	91.4	(44.3)
Gross benefits paid	(34.6)	(35.0)
Net benefit obligation at December 31	603.1	514.0	559.2
Change in plan assets:
Fair value of plan assets at January 1	485.9	458.8
Actual return on plan assets	32.1	61.2
Employer contributions	1.3	0.9
Plan participants' contributions	0	0
Gross benefits paid	(34.6)	(35.0)
Fair value of plan assets at December 31	484.7	485.9	458.8
Under funded status at December 31	(118.4)	(28.1)
Pension Plans, Defined Benefit [Member] \| WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	460.8	506.7
Service cost	4.9	5.9	5.2
Interest cost	22.6	20.7	21.6
Plan participants' contributions	0	0
Additional benefit costs	0	0.6
Actuarial (gain) loss	86.7	(41.1)
Gross benefits paid	(27.4)	(32.0)
Net benefit obligation at December 31	547.6	460.8	506.7
Change in plan assets:
Fair value of plan assets at January 1	438.8	415.4
Actual return on plan assets	28.6	55.2
Employer contributions	0.3	0.2
Plan participants' contributions	0	0
Gross benefits paid	(27.4)	(32.0)
Fair value of plan assets at December 31	440.3	438.8	415.4
Under funded status at December 31	(107.3)	(22.0)
Other Postretirement Benefits Plans [Member]
Change in benefit obligation:
Net benefit obligation at January 1	208.7	223.2
Service cost	5.2	6.3	6.9
Interest cost	9.5	8.5	10.2
Plan participants' contributions	2.8	2.6
Additional benefit costs	0	0
Actuarial (gain) loss	22.3	(13.2)
Gross benefits paid	(17.4)	(18.7)
Net benefit obligation at December 31	231.1	208.7	223.2
Change in plan assets:
Fair value of plan assets at January 1	124.9	123.1
Actual return on plan assets	5.6	14.4
Employer contributions	5.7	3.5
Plan participants' contributions	2.8	2.6
Gross benefits paid	(17.4)	(18.7)
Fair value of plan assets at December 31	121.6	124.9	123.1
Under funded status at December 31	(109.5)	(83.8)
Other Postretirement Benefits Plans [Member] \| IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	87.8	96.0
Service cost	2.4	2.9	3.0
Interest cost	3.9	3.6	4.4
Plan participants' contributions	0.9	0.9
Additional benefit costs	0	0
Actuarial (gain) loss	8.6	(7.0)
Gross benefits paid	(7.2)	(8.6)
Net benefit obligation at December 31	96.4	87.8	96.0
Change in plan assets:
Fair value of plan assets at January 1	81.2	78.8
Actual return on plan assets	3.6	10.0
Employer contributions	0.2	0.1
Plan participants' contributions	0.9	0.9
Gross benefits paid	(7.2)	(8.6)
Fair value of plan assets at December 31	78.7	81.2	78.8
Under funded status at December 31	(17.7)	(6.6)
Other Postretirement Benefits Plans [Member] \| WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	85.6	89.1
Service cost	2.0	2.5	2.7
Interest cost	3.8	3.4	4.1
Plan participants' contributions	1.3	1.2
Additional benefit costs	0	0
Actuarial (gain) loss	9.2	(3.0)
Gross benefits paid	(7.9)	(7.6)
Net benefit obligation at December 31	94.0	85.6	89.1
Change in plan assets:
Fair value of plan assets at January 1	21.7	22.3
Actual return on plan assets	1.2	2.5
Employer contributions	5.5	3.3
Plan participants' contributions	1.3	1.2
Gross benefits paid	(7.9)	(7.6)
Fair value of plan assets at December 31	21.8	21.7	22.3
Under funded status at December 31	$ (72.2)	$ (63.9)

Defined Benefit Plan, Additional Benefit Costs

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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)

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-Publisher FASB

-Name Accounting Standards Codification

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Actual Return on Plan Assets (Component of Net Periodic Pension Cost)

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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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-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

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-Glossary Gain or Loss

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-Glossary Gain or Loss

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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

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Projected Benefit Obligation

-URI http://asc.fasb.org/extlink&oid=6522206

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-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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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Postretirement Benefits

-URI http://asc.fasb.org/extlink&oid=6521376

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-Name Accounting Standards Codification

-Topic 715

-SubTopic 60

-Section 50

-Paragraph 4

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-Name Accounting Standards Codification

-Topic 715

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-Glossary Benefits

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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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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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- Definition

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.

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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.

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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.

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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.

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-Glossary Service Cost (Component of Net Periodic Pension Cost)

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