XML 57 R36.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Summary Of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Mar. 31, 2018 | Sep. 30, 2017 | Mar. 31, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Money market fund investments |  |  |  |  |  | $ 228.0 |  | $ 0.0
Money market fund investments interest rate, percentage |  |  |  |  |  | 2.00% |  |
Adjustment to net periodic benefit costs, percentage |  |  |  |  |  |  | 40.00% |
Income statement impact of adoption of Accounting Standards Update 2017-17 |  | $ 4.8 |  |  |  |  | $ 13.5 |
Cash receipts on sold receivables |  |  |  |  |  | $ 337.2 | 432.1 |
Net cash flows from operating activities | $ 156.3 |  | $ 173.5 | $ 274.4 | $ 249.8 | 442.2 | $ 451.3 |
IPL [Member] |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Money market fund investments |  |  |  |  |  | $ 228.0 |  | 0.0
Money market fund investments interest rate, percentage |  |  |  |  |  | 2.00% |  |
Adjustment to net periodic benefit costs, percentage |  |  |  |  |  |  | 40.00% |
Income statement impact of adoption of Accounting Standards Update 2017-17 |  | 2.0 |  |  |  |  | $ 5.5 |
Cash receipts on sold receivables |  |  |  |  |  | $ 337.2 | 432.1 |
Net cash flows from operating activities | $ 1.8 |  | $ 16.1 | $ 12.8 | $ 15.8 | 38.9 | $ 38.4 |
WPL [Member] |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Adjustment to net periodic benefit costs, percentage |  |  |  |  |  |  | 40.00% |
Income statement impact of adoption of Accounting Standards Update 2017-17 |  | $ 2.8 |  |  |  |  | $ 7.9 |
Net cash flows from operating activities |  |  |  |  |  | $ 345.5 | 361.2 |
Alliant Energy and IPL [Member] |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Cash receipts on sold receivables |  |  |  |  |  |  | $ 432.1 | $ 461.8

X
- Definition Adjustment To Net Periodic Benefit Costs, Percentage + References No definition available. + Details
Name: | lnt_AdjustmentToNetPeriodicBenefitCostsPercentage
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted Average Rate, Money Market Investment + References No definition available. + Details
Name: | lnt_WeightedAverageRateMoneyMarketInvestment
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + 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 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_CashEquivalentsAtCarryingValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of cash inflow (outflow) from operating activities, including discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 24 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3521-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585 + Details
Name: | us-gaap_NetCashProvidedByUsedInOperatingActivities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of the effect of a change in accounting principle on operating profit or loss. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b)(2) -URI http://asc.fasb.org/extlink&oid=64853466&loc=d3e22499-107794 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64853466&loc=d3e22580-107794 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=64853466&loc=d3e22583-107794 + Details
Name: | us-gaap_NewAccountingPronouncementOrChangeInAccountingPrincipleEffectOfChangeOnOperatingResults
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of cash inflow from collection on beneficial interest in securitization of receivables. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585 + Details
Name: | us-gaap_ProceedsFromCollectionOfRetainedInterestInSecuritizedReceivables
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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: |

X
- Details
Name: | dei_LegalEntityAxis=lnt_AlliantEnergyAndIplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |