Benefit Plans (Funded Status of Benefits Plans) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plans, Defined Benefit [Member]
Change in benefit obligation:
Net benefit obligation at January 1	$ 1,303.1	$ 1,244.3
Service cost	12.1	12.5	$ 12.6
Interest cost	46.8	51.0	53.0
Plan participants' contributions	0.0	0.0
Actuarial (gain) loss	(96.2)	83.6
Gross benefits paid	(90.8)	(88.3)
Net benefit obligation at December 31	1,175.0	1,303.1	1,244.3
Change in plan assets:
Fair value of plan assets at January 1	950.7	895.7
Actual return on plan assets	(57.8)	136.7
Employer contributions	6.5	6.6
Plan participants' contributions	0.0	0.0
Gross benefits paid	(90.8)	(88.3)
Fair value of plan assets at December 31	808.6	950.7	895.7
Under funded status at December 31	(366.4)	(352.4)
Pension Plans, Defined Benefit [Member] \| IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	592.9	570.4
Service cost	7.4	7.3	7.5
Interest cost	21.4	23.5	24.5
Plan participants' contributions	0.0	0.0
Actuarial (gain) loss	(44.4)	34.9
Gross benefits paid	(43.4)	(43.2)
Net benefit obligation at December 31	533.9	592.9	570.4
Change in plan assets:
Fair value of plan assets at January 1	443.7	422.0
Actual return on plan assets	(26.8)	64.3
Employer contributions	4.3	0.6
Plan participants' contributions	0.0	0.0
Gross benefits paid	(43.4)	(43.2)
Fair value of plan assets at December 31	377.8	443.7	422.0
Under funded status at December 31	(156.1)	(149.2)
Pension Plans, Defined Benefit [Member] \| WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	559.8	529.2
Service cost	4.4	4.9	4.9
Interest cost	20.2	21.8	22.3
Plan participants' contributions	0.0	0.0
Actuarial (gain) loss	(40.0)	38.3
Gross benefits paid	(37.8)	(34.4)
Net benefit obligation at December 31	506.6	559.8	529.2
Change in plan assets:
Fair value of plan assets at January 1	415.0	389.7
Actual return on plan assets	(25.1)	59.6
Employer contributions	0.1	0.1
Plan participants' contributions	0.0	0.0
Gross benefits paid	(37.8)	(34.4)
Fair value of plan assets at December 31	352.2	415.0	389.7
Under funded status at December 31	(154.4)	(144.8)
Other Postretirement Benefits Plans [Member]
Change in benefit obligation:
Net benefit obligation at January 1	222.3	220.1
Service cost	4.2	5.0	5.3
Interest cost	7.7	8.6	9.4
Plan participants' contributions	3.1	2.9
Actuarial (gain) loss	(8.2)	5.4
Gross benefits paid	(23.0)	(19.7)
Net benefit obligation at December 31	206.1	222.3	220.1
Change in plan assets:
Fair value of plan assets at January 1	111.1	105.8
Actual return on plan assets	(2.6)	12.9
Employer contributions	10.5	9.2
Plan participants' contributions	3.1	2.9
Gross benefits paid	(23.0)	(19.7)
Fair value of plan assets at December 31	99.1	111.1	105.8
Under funded status at December 31	(107.0)	(111.2)
Other Postretirement Benefits Plans [Member] \| IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	89.4	90.1
Service cost	1.7	2.1	2.3
Interest cost	3.1	3.5	3.8
Plan participants' contributions	1.0	1.0
Actuarial (gain) loss	(4.3)	(0.1)
Gross benefits paid	(8.4)	(7.2)
Net benefit obligation at December 31	82.5	89.4	90.1
Change in plan assets:
Fair value of plan assets at January 1	72.9	68.2
Actual return on plan assets	(1.4)	8.9
Employer contributions	2.6	2.0
Plan participants' contributions	1.0	1.0
Gross benefits paid	(8.4)	(7.2)
Fair value of plan assets at December 31	66.7	72.9	68.2
Under funded status at December 31	(15.8)	(16.5)
Other Postretirement Benefits Plans [Member] \| WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	90.4	88.9
Service cost	1.6	1.9	2.0
Interest cost	3.1	3.4	3.8
Plan participants' contributions	1.4	1.4
Actuarial (gain) loss	(2.5)	4.1
Gross benefits paid	(10.8)	(9.3)
Net benefit obligation at December 31	83.2	90.4	88.9
Change in plan assets:
Fair value of plan assets at January 1	18.7	18.6
Actual return on plan assets	(0.1)	1.2
Employer contributions	7.5	6.8
Plan participants' contributions	1.4	1.4
Gross benefits paid	(10.8)	(9.3)
Fair value of plan assets at December 31	16.7	18.7	$ 18.6
Under funded status at December 31	$ (66.5)	$ (71.7)

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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- Definition

Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

+ References

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- Definition

Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

+ References

+ Details

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- Definition

Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 60
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(6)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount of contributions received by defined benefit plan from participant which increase benefit obligation.

+ 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.

+ Details

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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.

+ Details

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- Definition

Amount of contribution received by defined benefit plan from employer which increases plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(ii)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

+ Details

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- Definition

Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

+ References

+ Details

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- Definition

Amount of cost recognized for passage of time related to defined benefit plan.

+ References

+ Details

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- Definition

Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

+ References

+ Details

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