Regulatory Matters (Revenue Requirement Adjustment) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Regulatory Liabilities [Line Items]
Electric revenues	$ 2,713.6	$ 2,689.0	$ 2,589.3
Alliant Energy and IPL [Member] \| Revenue Requirement Adjustment [Member]
Regulatory Liabilities [Line Items]
Electric revenues	$ 15.0	$ 24.0

Revenue derived from the regulated (by a federal, state, or local government or agency) generation, transmission and distribution of electricity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1(b))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Name:	us-gaap_ElectricDomesticRegulatedRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration