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Receivables (Narrative) (Details) - Receivables Sold [Member] - IPL [Member] $ in Millions	Jun. 30, 2017 USD ($)
Receivables [Line Items]
Available capacity	$ 36.9
Outstanding receivables past due	$ 53.1

X

- Definition

Outstanding receivables past due

+ References

No definition available.

+ Details

Name:	lnt_OutstandingReceivablesPastDue
Namespace Prefix:	lnt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Sale of Accounts Receivable Program, Capacity Available

+ References

No definition available.

+ Details

Name:	lnt_SaleofAccountsReceivableProgramCapacityAvailable
Namespace Prefix:	lnt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

X

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=lnt_ReceivablesSoldMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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