Fair Value Measurements (Fair Value Of Net Derivative Assets (Liabilities)) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	$ (10.7)	$ (29.4)	$ (12.2)	$ 9.2	$ (32.9)	$ 8.7
Excluding Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative liabilities	29.9		23.5
Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	19.2		11.3
IPL [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	(4.1)	(15.4)	(1.4)	17.1	(8.3)	10.1
IPL [Member] | Excluding Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative liabilities	18.1		11.5
IPL [Member] | Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	14.0		10.1
WPL [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	(6.6)	$ (14.0)	(10.8)	$ (7.9)	$ (24.6)	$ (1.4)
WPL [Member] | Excluding Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative liabilities	11.8		12.0
WPL [Member] | Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	$ 5.2		$ 1.2