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Income Taxes (Narrative) (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2017
USD ($)
Income Taxes [Line Items]  
Increase in deferred tax liabilities $ 55.7
IPL [Member]  
Income Taxes [Line Items]  
Increase in deferred tax liabilities 41.4
WPL [Member]  
Income Taxes [Line Items]  
Increase in deferred tax liabilities $ 13.1