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Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Commercial paper:
Amount outstanding	$ 41.5us-gaap_CommercialPaper	$ 141.3us-gaap_CommercialPaper
Weighted average remaining maturity (in days)	1 day
Weighted average interest rates	0.50%
Available credit facility capacity	958.5
Parent Company [Member]
Commercial paper:
Amount outstanding	41.5
Weighted average remaining maturity (in days)	1 day
Weighted average interest rates	0.50%
Available credit facility capacity	258.5
IPL [Member]
Commercial paper:
Amount outstanding	0
Available credit facility capacity	300.0
WPL [Member]
Commercial paper:
Amount outstanding	0
Available credit facility capacity	$ 400.0

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

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