Common Equity (Changes in Shareowners' Equity) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance		$ 5,205.1		$ 5,205.1
Beginning balance	$ 5,702.4	5,405.1	$ 4,882.4	5,405.1	$ 4,785.7
Net income attributable to common shareowners	134.4		94.6	304.4	219.7
Net income	136.9		97.1	309.5	224.8
Common stock dividends	(94.6)		(84.1)	(187.6)	(167.8)
Equity forward settlements and Shareowner Direct Plan issuances	6.4		6.0	234.8	60.6
Equity-based compensation plans and other	2.3		2.1	2.2	2.1
Adoption of new accounting standard, net of tax				(8.7)
Other comprehensive loss, net of tax	(2.6)		(1.4)	(1.9)	(0.7)
Ending balance	5,548.3			5,548.3
Ending balance	$ 5,748.3	5,702.4	$ 4,899.6	$ 5,748.3	$ 4,899.6
Common stock dividends (in dollars per share)	$ 0.38		$ 0.355	$ 0.76	$ 0.71
Common Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	$ 2.5	2.5	$ 2.4	$ 2.5	$ 2.4
Equity forward settlements and Shareowner Direct Plan issuances
Ending balance	2.5	2.5	2.4	2.5	2.4
Additional Paid-in Capital [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	2,674.1	2,445.9	2,100.0	2,445.9	2,045.5
Equity forward settlements and Shareowner Direct Plan issuances	6.4		6.0	234.8	60.6
Equity-based compensation plans and other	2.6		2.4	2.4	2.3
Ending balance	2,683.1	2,674.1	2,108.4	2,683.1	2,108.4
Retained Earnings [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	2,833.7	2,765.4	2,587.3	2,765.4	2,545.9
Net income attributable to common shareowners	134.4		94.6	304.4	219.7
Common stock dividends	(94.6)		(84.1)	(187.6)	(167.8)
Ending balance	2,873.5	2,833.7	2,597.8	2,873.5	2,597.8
Accumulated Other Comprehensive Income (Loss) [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	2.0	1.3	2.4	1.3	1.7
Other comprehensive loss, net of tax	(2.6)		(1.4)	(1.9)	(0.7)
Ending balance	(0.6)	2.0	1.0	(0.6)	1.0
Shares in Deferred Compensation Trust [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	(9.9)	(10.0)	(9.7)	(10.0)	(9.8)
Equity-based compensation plans and other	(0.3)		(0.3)	(0.2)	(0.2)
Ending balance	(10.2)	(9.9)	(10.0)	(10.2)	(10.0)
Cumulative Preferred Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	200.0	200.0	200.0	200.0	200.0
Ending balance	200.0	200.0	200.0	200.0	200.0
IPL [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance		3,271.8		3,271.8
Beginning balance	3,595.4	3,471.8	3,342.0	3,471.8	3,230.7
Net income attributable to common shareowners	59.8		45.0	142.4	98.3
Net income	62.3		47.5	147.5	103.4
Common stock dividends	(59.1)		(41.9)	(118.1)	(83.9)
Capital contributions from parent	75.0			175.0	100.0
Ending balance	3,471.1			3,471.1
Ending balance	3,671.1	3,595.4	3,345.1	3,671.1	3,345.1
IPL [Member] \| Common Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	33.4	33.4	33.4	33.4	33.4
Ending balance	33.4	33.4	33.4	33.4	33.4
IPL [Member] \| Additional Paid-in Capital [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	2,447.8	2,347.8	2,322.8	2,347.8	2,222.8
Capital contributions from parent	75.0			175.0	100.0
Ending balance	2,522.8	2,447.8	2,322.8	2,522.8	2,322.8
IPL [Member] \| Retained Earnings [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	914.2	890.6	785.8	890.6	774.5
Net income attributable to common shareowners	59.8		45.0	142.4	98.3
Common stock dividends	(59.1)		(41.9)	(118.1)	(83.9)
Ending balance	914.9	914.2	788.9	914.9	788.9
IPL [Member] \| Cumulative Preferred Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	200.0	200.0	200.0	200.0	200.0
Ending balance	200.0	200.0	200.0	200.0	200.0
WPL [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance		2,363.6		2,363.6
Beginning balance	2,436.1	2,363.6	2,179.2	2,363.6	2,149.5
Net income	57.6		42.0	147.2	107.7
Common stock dividends	(41.9)		(36.0)	(84.0)	(72.0)
Capital contributions from parent				25.0	0.0
Ending balance	2,451.8			2,451.8
Ending balance	2,451.8	2,436.1	2,185.2	2,451.8	2,185.2
WPL [Member] \| Common Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	66.2	66.2	66.2	66.2	66.2
Ending balance	66.2	66.2	66.2	66.2	66.2
WPL [Member] \| Additional Paid-in Capital [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	1,459.0	1,434.0	1,309.0	1,434.0	1,309.0
Capital contributions from parent				25.0
Ending balance	1,459.0	1,459.0	1,309.0	1,459.0	1,309.0
WPL [Member] \| Retained Earnings [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	910.9	863.4	804.0	863.4	774.3
Net income	57.6		42.0	147.2	107.7
Common stock dividends	(41.9)		(36.0)	(84.0)	(72.0)
Ending balance	$ 926.6	$ 910.9	$ 810.0	926.6	$ 810.0
Credit Losses ASU [Member] \| Retained Earnings [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Adoption of new accounting standard, net of tax				$ (8.7)

Aggregate dividends declared during the period for each share of common stock outstanding.

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Amount of the cumulative effect on retained earnings net of related income tax effect.

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