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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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