Debt (Credit Facilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Commercial paper:
Amount outstanding	$ 320.2	$ 244.1
Weighted average interest rates	2.00%	0.90%
Available credit facility capacity	$ 679.8	$ 755.9
IPL [Member]
Commercial paper:
Amount outstanding	0.0	0.0
Available credit facility capacity	250.0	300.0
WPL [Member]
Commercial paper:
Amount outstanding	$ 25.0	$ 52.3
Weighted average interest rates	1.50%	0.70%
Available credit facility capacity	$ 325.0	$ 347.7

Commercial paper [Abstract]

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a)(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(2))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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Balance Type:	credit
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- Definition

Weighted average interest rate of short-term debt outstanding.

+ References

+ Details

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- Details

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- Details

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