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Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 397.6	$ 229.9	$ 397.6	$ 242.6
Average amount outstanding (based on daily outstanding balances)	$ 307.8	$ 213.0	$ 292.3	$ 206.0
Weighted average interest rates	1.10%	0.60%	1.00%	0.60%
IPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 14.6	$ 1.7	$ 14.6	$ 1.7
Average amount outstanding (based on daily outstanding balances)	$ 1.0	$ 0.0	$ 0.6	$ 0.0
Weighted average interest rates	1.20%	0.60%	1.20%	0.60%
WPL [Member]
Debt [Line Items]
Maximum amount outstanding (based on daily outstanding balances)	$ 212.5	$ 62.9	$ 212.5	$ 62.9
Average amount outstanding (based on daily outstanding balances)	$ 134.9	$ 37.4	$ 107.2	$ 31.6
Weighted average interest rates	1.00%	0.40%	0.90%	0.40%

X

- Definition

Reflects the calculation as of the balance sheet date of the average interest rate weighted by the amount of short-term debt outstanding by type or by instrument at that time.

+ References

No definition available.

+ Details

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Namespace Prefix:	lnt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, average borrowings during the period.

+ References

No definition available.

+ Details

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

X

- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, the maximum amount borrowed at any time during the period.

+ References

No definition available.

+ Details

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

X

- Details

Name:	dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=lnt_WplMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: