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Common Equity (Changes in Shareowners' Equity) (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Changes in Shareowners' Equity Roll Forward [Line Items] |  |  |  |
Beginning balance |  |  | $ 4,585.7 |
Beginning balance | $ 4,899.6 | $ 4,550.7 | 4,785.7 | $ 4,382.2
Net income attributable to common shareowners | 226.0 | 205.5 | 445.7 | 426.8
Common stock dividends | (84.7) | (78.5) | (252.5) | (233.3)
Equity forward settlements, At-the-market offering program and Shareowner Direct Plan issuances | 124.8 | 91.2 | 185.4 | 191.3
Equity-based compensation plans and other | 3.5 | 1.8 | 5.6 | 3.1
Other comprehensive income (loss), net of tax |  | (0.3) | (0.7) | 0.3
Ending balance | 4,969.2 |  | 4,969.2 |
Ending balance | $ 5,169.2 | $ 4,770.4 | $ 5,169.2 | $ 4,770.4
Common stock dividends (in dollars per share) | $ 0.355 | $ 0.335 | $ 1.065 | $ 1.005
Common Stock [Member] |  |  |  |
Changes in Shareowners' Equity Roll Forward [Line Items] |  |  |  |
Beginning balance | $ 2.4 | $ 2.3 | $ 2.4 | $ 2.3
Equity forward settlements, At-the-market offering program and Shareowner Direct Plan issuances |  | 0.1 |  | 0.1
Ending balance | 2.4 | 2.4 | 2.4 | 2.4
Additional Paid-in Capital [Member] |  |  |  |
Changes in Shareowners' Equity Roll Forward [Line Items] |  |  |  |
Beginning balance | 2,108.4 | 1,947.2 | 2,045.5 | 1,845.5
Equity forward settlements, At-the-market offering program and Shareowner Direct Plan issuances | 124.8 | 91.1 | 185.4 | 191.2
Equity-based compensation plans and other | 3.7 | (0.1) | 6.0 | 1.5
Ending balance | 2,236.9 | 2,038.2 | 2,236.9 | 2,038.2
Retained Earnings [Member] |  |  |  |
Changes in Shareowners' Equity Roll Forward [Line Items] |  |  |  |
Beginning balance | 2,597.8 | 2,412.5 | 2,545.9 | 2,346.0
Net income attributable to common shareowners | 226.0 | 205.5 | 445.7 | 426.8
Common stock dividends | (84.7) | (78.5) | (252.5) | (233.3)
Ending balance | 2,739.1 | 2,539.5 | 2,739.1 | 2,539.5
Accumulated Other Comprehensive Income (Loss) [Member] |  |  |  |
Changes in Shareowners' Equity Roll Forward [Line Items] |  |  |  |
Beginning balance | 1.0 | 0.1 | 1.7 | (0.5)
Other comprehensive income (loss), net of tax |  | (0.3) | (0.7) | 0.3
Ending balance | 1.0 | (0.2) | 1.0 | (0.2)
Shares in Deferred Compensation Trust [Member] |  |  |  |
Changes in Shareowners' Equity Roll Forward [Line Items] |  |  |  |
Beginning balance | (10.0) | (11.4) | (9.8) | (11.1)
Equity-based compensation plans and other | (0.2) | 1.9 | (0.4) | 1.6
Ending balance | (10.2) | (9.5) | (10.2) | (9.5)
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 | 3,345.1 | 2,854.1 | 3,230.7 | 2,709.7
Net income attributable to common shareowners | 141.1 | 126.5 | 239.4 | 224.9
Common stock dividends | (42.1) | (41.9) | (126.0) | (125.9)
Capital contributions from parent |  | 100.0 | 100.0 | 230.0
Other comprehensive income (loss), net of tax | 0.0 | 0.0 | 0.0 | 0.0
Ending balance | 3,244.1 |  | 3,244.1 |
Ending balance | 3,444.1 | 3,038.7 | 3,444.1 | 3,038.7
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 | 1,927.8 | 2,222.8 | 1,797.8
Capital contributions from parent |  | 100.0 | 100.0 | 230.0
Ending balance | 2,322.8 | 2,027.8 | 2,322.8 | 2,027.8
IPL [Member] | Retained Earnings [Member] |  |  |  |
Changes in Shareowners' Equity Roll Forward [Line Items] |  |  |  |
Beginning balance | 788.9 | 692.9 | 774.5 | 678.5
Net income attributable to common shareowners | 141.1 | 126.5 | 239.4 | 224.9
Common stock dividends | (42.1) | (41.9) | (126.0) | (125.9)
Ending balance | 887.9 | 777.5 | 887.9 | 777.5
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 | 2,185.2 | 2,055.2 | 2,149.5 | 1,881.5
Net income attributable to common shareowners | 75.5 | 76.3 | 183.2 | 170.1
Common stock dividends | (35.9) | (34.9) | (107.9) | (105.0)
Capital contributions from parent | 100.0 | 50.0 | 100.0 | 200.0
Other comprehensive income (loss), net of tax | 0.0 | 0.0 | 0.0 | 0.0
Ending balance | 2,324.8 |  | 2,324.8 |
Ending balance | 2,324.8 | 2,146.6 | 2,324.8 | 2,146.6
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 | 1,259.0 | 1,309.0 | 1,109.0
Capital contributions from parent | 100.0 | 50.0 | 100.0 | 200.0
Ending balance | 1,409.0 | 1,309.0 | 1,409.0 | 1,309.0
WPL [Member] | Retained Earnings [Member] |  |  |  |
Changes in Shareowners' Equity Roll Forward [Line Items] |  |  |  |
Beginning balance | 810.0 | 730.0 | 774.3 | 706.3
Net income attributable to common shareowners | 75.5 | 76.3 | 183.2 | 170.1
Common stock dividends | (35.9) | (34.9) | (107.9) | (105.0)
Ending balance | $ 849.6 | $ 771.4 | $ 849.6 | $ 771.4

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