Common Equity (Changes in Shareowners' Equity) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance			$ 4,585.7
Beginning balance	$ 4,882.4	$ 4,431.5	4,785.7	$ 4,382.2
Net income attributable to common shareowners	94.6	100.4	219.7	221.3
Common stock dividends	(84.1)	(77.3)	(167.8)	(154.8)
Equity forward settlements, At-the-market Offering Program and Shareowner Direct Plan issuances	6.0	93.8	60.6	100.1
Equity-based compensation plans and other	2.1	1.7	2.1	1.3
Other comprehensive income (loss), net of tax	(1.4)	0.6	(0.7)	0.6
Ending balance	4,699.6		4,699.6
Ending balance	$ 4,899.6	$ 4,550.7	$ 4,899.6	$ 4,550.7
Common stock dividends (in dollars per share)	$ 0.355	$ 0.335	$ 0.71	$ 0.67
Common Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	$ 2.4	$ 2.3	$ 2.4	$ 2.3
Ending balance	2.4	2.3	2.4	2.3
Additional Paid-in Capital [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	2,100.0	1,851.4	2,045.5	1,845.5
Equity forward settlements, At-the-market Offering Program and Shareowner Direct Plan issuances	6.0	93.8	60.6	100.1
Equity-based compensation plans and other	2.4	2.0	2.3	1.6
Ending balance	2,108.4	1,947.2	2,108.4	1,947.2
Retained Earnings [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	2,587.3	2,389.4	2,545.9	2,346.0
Net income attributable to common shareowners	94.6	100.4	219.7	221.3
Common stock dividends	(84.1)	(77.3)	(167.8)	(154.8)
Ending balance	2,597.8	2,412.5	2,597.8	2,412.5
Accumulated Other Comprehensive Income (Loss) [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	2.4	(0.5)	1.7	(0.5)
Other comprehensive income (loss), net of tax	(1.4)	0.6	(0.7)	0.6
Ending balance	1.0	0.1	1.0	0.1
Shares in Deferred Compensation Trust [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	(9.7)	(11.1)	(9.8)	(11.1)
Equity-based compensation plans and other	(0.3)	(0.3)	(0.2)	(0.3)
Ending balance	(10.0)	(11.4)	(10.0)	(11.4)
Cumulative Preferred Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	200.0	200.0	200.0	200.0
Ending balance	200.0	200.0	200.0	200.0
IPL [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance			3,030.7
Beginning balance	3,342.0	2,714.5	3,230.7	2,709.7
Net income attributable to common shareowners	45.0	51.7	98.3	98.4
Common stock dividends	(41.9)	(42.1)	(83.9)	(84.0)
Capital contributions from parent		130.0	100.0	130.0
Other comprehensive income (loss), net of tax	0.0	0.0	0.0	0.0
Ending balance	3,145.1		3,145.1
Ending balance	3,345.1	2,854.1	3,345.1	2,854.1
IPL [Member] \| Common Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	33.4	33.4	33.4	33.4
Ending balance	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	2,322.8	1,797.8	2,222.8	1,797.8
Capital contributions from parent		130.0	100.0	130.0
Ending balance	2,322.8	1,927.8	2,322.8	1,927.8
IPL [Member] \| Retained Earnings [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	785.8	683.3	774.5	678.5
Net income attributable to common shareowners	45.0	51.7	98.3	98.4
Common stock dividends	(41.9)	(42.1)	(83.9)	(84.0)
Ending balance	788.9	692.9	788.9	692.9
IPL [Member] \| Cumulative Preferred Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	200.0	200.0	200.0	200.0
Ending balance	200.0	200.0	200.0	200.0
WPL [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance			2,149.5
Beginning balance	2,179.2	1,900.5	2,149.5	1,881.5
Net income attributable to common shareowners	42.0	39.8	107.7	93.8
Common stock dividends	(36.0)	(35.1)	(72.0)	(70.1)
Capital contributions from parent		150.0	0.0	150.0
Other comprehensive income (loss), net of tax	0.0	0.0	0.0	0.0
Ending balance	2,185.2		2,185.2
Ending balance	2,185.2	2,055.2	2,185.2	2,055.2
WPL [Member] \| Common Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	66.2	66.2	66.2	66.2
Ending balance	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	1,309.0	1,109.0	1,309.0	1,109.0
Capital contributions from parent		150.0		150.0
Ending balance	1,309.0	1,259.0	1,309.0	1,259.0
WPL [Member] \| Retained Earnings [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	804.0	725.3	774.3	706.3
Net income attributable to common shareowners	42.0	39.8	107.7	93.8
Common stock dividends	(36.0)	(35.1)	(72.0)	(70.1)
Ending balance	$ 810.0	$ 730.0	$ 810.0	$ 730.0

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