Regulatory Matters (Gas Tax Benefit Rider) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	$ (67.8)	$ 10.8	$ (90.8)
Income tax expense (benefit)	70.4	44.3	53.9
Gas [Member] \| IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(12.0)
Alliant Energy and IPL [Member] \| Gas [Member] \| IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Increase (decrease) in regulatory liabilities	(12.0)	(12.0)	(11.0)
Income tax expense (benefit)	(7.0)	(7.0)	(7.0)
Alliant Energy and IPL [Member] \| Gas [Member] \| IPL's Tax Benefit Rider Decrease In Taxable Income [Member]
Regulatory Liabilities [Line Items]
Income tax expense (benefit)	$ (5.0)	$ (5.0)	$ (4.0)

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a),(b)
-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Income Tax Expense (or Benefit)
-URI http://asc.fasb.org/extlink&oid=6515339

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 08
-Paragraph h
-Article 4

Name:	us-gaap_IncomeTaxExpenseBenefit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration