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Benefit Plans (Recognized Compensation Expense And Income Tax Benefits) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Compensation expense | $ 15.3 us-gaap_AllocatedShareBasedCompensationExpense | $ 12.0 us-gaap_AllocatedShareBasedCompensationExpense | $ 6.9 us-gaap_AllocatedShareBasedCompensationExpense
Income tax benefits | 6.2 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 4.8 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 2.8 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
IPL [Member] |  |  |
Compensation expense | 8.3 us-gaap_AllocatedShareBasedCompensationExpense / dei_LegalEntityAxis = lnt_IplMember | 6.2 us-gaap_AllocatedShareBasedCompensationExpense / dei_LegalEntityAxis = lnt_IplMember | 3.6 us-gaap_AllocatedShareBasedCompensationExpense / dei_LegalEntityAxis = lnt_IplMember
Income tax benefits | 3.4 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / dei_LegalEntityAxis = lnt_IplMember | 2.5 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / dei_LegalEntityAxis = lnt_IplMember | 1.5 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |  |
Compensation expense | 6.4 us-gaap_AllocatedShareBasedCompensationExpense / dei_LegalEntityAxis = lnt_WplMember | 5.2 us-gaap_AllocatedShareBasedCompensationExpense / dei_LegalEntityAxis = lnt_WplMember | 3.0 us-gaap_AllocatedShareBasedCompensationExpense / dei_LegalEntityAxis = lnt_WplMember
Income tax benefits | $ 2.6 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / dei_LegalEntityAxis = lnt_WplMember | $ 2.1 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / dei_LegalEntityAxis = lnt_WplMember | $ 1.2 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / dei_LegalEntityAxis = lnt_WplMember

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | duration

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- Details
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- Details
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