Fair Value Measurements Fair Value Measurements (Fair Value Of Net Derivative Assets (Liabilities)) (Details) (Commodity Contracts [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	$ 68.3	$ 101.2	$ 4.4	$ 17.2	$ 42.5	$ 11.9
Excluding Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	29.7
Fair value, net derivative liabilities			(13.9)
Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	38.6		18.3
IPL [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	36.5	64.2	14.6	28.6	40.6	12.5
IPL [Member] | Excluding Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	3.3
Fair value, net derivative liabilities			(2.1)
IPL [Member] | Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	33.2		16.7
WPL [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	31.8	37.0	(10.2)	(11.4)	1.9	(0.6)
WPL [Member] | Excluding Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	26.4
Fair value, net derivative liabilities			(11.8)
WPL [Member] | Financial Transmission Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, net derivative assets	$ 5.4		$ 1.6