Common Equity (Changes in Shareowners' Equity) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	$ 4,585.7
Beginning balance	4,785.7	$ 4,382.2
Net income attributable to common shareowners	125.1	120.9
Common stock dividends	(83.7)	(77.5)
Equity forward settlements and Shareowner Direct Plan issuances	54.6	6.3
Equity-based compensation plans and other	0.0	(0.4)
Other comprehensive income, net of tax	0.7
Ending balance	4,682.4
Ending balance	$ 4,882.4	$ 4,431.5
Common stock dividends (in dollars per share)	$ 0.355	$ 0.335
Common Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	$ 2.4	$ 2.3
Ending balance	2.4	2.3
Additional Paid-in Capital [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	2,045.5	1,845.5
Equity forward settlements and Shareowner Direct Plan issuances	54.6	6.3
Equity-based compensation plans and other	(0.1)	(0.4)
Ending balance	2,100.0	1,851.4
Retained Earnings [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	2,545.9	2,346.0
Net income attributable to common shareowners	125.1	120.9
Common stock dividends	(83.7)	(77.5)
Ending balance	2,587.3	2,389.4
Accumulated Other Comprehensive Income (Loss) [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	1.7	(0.5)
Other comprehensive income, net of tax	0.7
Ending balance	2.4	(0.5)
Shares in Deferred Compensation Trust [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	(9.8)	(11.1)
Equity-based compensation plans and other	0.1
Ending balance	(9.7)	(11.1)
Cumulative Preferred Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	200.0	200.0
Ending balance	200.0	200.0
IPL [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	3,030.7
Beginning balance	3,230.7	2,709.7
Net income attributable to common shareowners	53.3	46.7
Common stock dividends	(42.0)	(41.9)
Capital contributions from parent	100.0	0.0
Other comprehensive income, net of tax	0.0	0.0
Ending balance	3,142.0
Ending balance	3,342.0	2,714.5
IPL [Member] | Common Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	33.4	33.4
Ending balance	33.4	33.4
IPL [Member] | Additional Paid-in Capital [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	2,222.8	1,797.8
Capital contributions from parent	100.0
Ending balance	2,322.8	1,797.8
IPL [Member] | Retained Earnings [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	774.5	678.5
Net income attributable to common shareowners	53.3	46.7
Common stock dividends	(42.0)	(41.9)
Ending balance	785.8	683.3
IPL [Member] | Cumulative Preferred Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	200.0	200.0
Ending balance	200.0	200.0
WPL [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	2,149.5
Beginning balance	2,149.5	1,881.5
Net income attributable to common shareowners	65.7	54.0
Common stock dividends	(36.0)	(35.0)
Other comprehensive income, net of tax	0.0	0.0
Ending balance	2,179.2
Ending balance	2,179.2	1,900.5
WPL [Member] | Common Stock [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	66.2	66.2
Ending balance	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
Ending balance	1,309.0	1,109.0
WPL [Member] | Retained Earnings [Member]
Changes in Shareowners' Equity Roll Forward [Line Items]
Beginning balance	774.3	706.3
Net income attributable to common shareowners	65.7	54.0
Common stock dividends	(36.0)	(35.0)
Ending balance	$ 804.0	$ 725.3