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Benefit Plans (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Benefit Plans [Line Items] |  |
Total plan assets, percentage of common stock (less than 1%) | 1.00% | 1.00%
Common stock percentage in assets held in 401(k) saving plans | 11.50% | 12.60%
Unrecognized compensation cost | $ 5.6 |
Percentage of base salary and performance-based compensation | 100.00% |
Carrying value of deferred compensation obligations | $ 21.8 | $ 19.4
Performance Shares and Performance Units [Member] |  |
Benefit Plans [Line Items] |  |
Performance period | 3 years |
Actual number of shares paid out upon vesting, maximum percentage of target shares | 200.00% |
Actual number of shares paid out upon vesting, minimum percentage of target shares | 0.00% |
Performance Shares [Member] |  |
Benefit Plans [Line Items] |  |
Instrument valuation based on shares of common stock, number of shares | 1 |
Performance Units [Member] |  |
Benefit Plans [Line Items] |  |
Instrument valuation based on shares of common stock, number of shares | 1 |
Performance Restricted Stock Unit And Performance Restricted Unit [Member] |  |
Benefit Plans [Line Items] |  |
Performance period | 3 years |
Actual number of shares paid out upon vesting, maximum percentage of target shares | 200.00% |
Actual number of shares paid out upon vesting, minimum percentage of target shares | 0.00% |
Performance Restricted Stock Unit [Member] |  |
Benefit Plans [Line Items] |  |
Instrument valuation based on shares of common stock, number of shares | 1 |
Restricted Stock Unit And Restricted Unit [Member] |  |
Benefit Plans [Line Items] |  |
Performance period | 3 years |
Instrument valuation based on shares of common stock, number of shares | 1 |
Minimum [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost recognized over a weighted average period | 1 year |
Maximum [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost recognized over a weighted average period | 2 years |
IPL [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost | $ 3.1 |
WPL [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost | 2.2 |
Other Postretirement Benefits Plans [Member] |  |
Benefit Plans [Line Items] |  |
Plan asset threshold for long-term allocation targets | 5.0 |
Other Postretirement Benefits Plans [Member] | IPL [Member] |  |
Benefit Plans [Line Items] |  |
Plan asset threshold for long-term allocation targets | 5.0 |
Other Postretirement Benefits Plans [Member] | WPL [Member] |  |
Benefit Plans [Line Items] |  |
Plan asset threshold for long-term allocation targets | $ 5.0 |
Omnibus Incentive Plan [Member] |  |
Benefit Plans [Line Items] |  |
Shares available for issuance under the Amended and Restated OIP | 7,100,000 |
2015 Grant [Member] | Performance Units [Member] |  |
Benefit Plans [Line Items] |  |
Instrument valuation based on shares of common stock, number of shares | 1 |
2016 Grant [Member] | Performance Units [Member] |  |
Benefit Plans [Line Items] |  |
Instrument valuation based on shares of common stock, number of shares | 1 |

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