Carrying amount as of the balance sheet date of capitalized fuel costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499739&loc=d3e43603-110378

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 13.A.4(a).Q1 Response))
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

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Amount of public utility's approved rate increase (decrease) by regulatory agency.

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Regulatory Matters (Retail Fuel-related Rate Filings) (Details) - WPL [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
2016 Test Year Retail Electric Fuel Related [Member]
Regulatory Liabilities [Line Items]
Authorized increase (decrease) in final rates, amount	$ 7
Authorized increase (decrease) in final rates, percent	1.00%
Retail electric fuel-related costs	$ 9
2015 Test Year Retail Electric Fuel Related [Member]
Regulatory Liabilities [Line Items]
Authorized increase (decrease) in final rates, amount		$ 39
Authorized increase (decrease) in final rates, percent		4.00%
Refund of over-collected fuel-related costs	10
2014 Test Year Retail Electric Fuel Related [Member]
Regulatory Liabilities [Line Items]
Authorized increase (decrease) in final rates, amount			$ 19
Authorized increase (decrease) in final rates, percent			2.00%
Deferral [Member] \| 2014 Test Year Retail Electric Fuel Related [Member]
Regulatory Liabilities [Line Items]
Authorized increase (decrease) in final rates, amount	$ (28)