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Segment Information (Details) (USD $)
3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Debt Instrument [Line Items]      
Cash $ 5,700,000us-gaap_Cash   5,900,000us-gaap_Cash
Long-term nonutility investments 5,100,000us-gaap_OtherLongTermInvestments   5,100,000us-gaap_OtherLongTermInvestments
Net income 22,209,000us-gaap_NetIncomeLoss 23,706,000us-gaap_NetIncomeLoss  
7.25% Senior Secured Notes [Member]      
Debt Instrument [Line Items]      
Senior secured note 400,000,000us-gaap_SeniorNotes
/ us-gaap_DebtInstrumentAxis
= cik0000728391_SevenPointTwoFivePercentSeniorSecuredNotesMember
   
Senior notes, interest rate, percentage 7.25%us-gaap_DebtInstrumentInterestRateEffectivePercentage
/ us-gaap_DebtInstrumentAxis
= cik0000728391_SevenPointTwoFivePercentSeniorSecuredNotesMember
   
Debt instrument, maturity date Apr. 01, 2016    
5.00% Senior Secured Notes [Member]      
Debt Instrument [Line Items]      
Senior secured note 400,000,000us-gaap_SeniorNotes
/ us-gaap_DebtInstrumentAxis
= cik0000728391_FivePointZeroZeroPercentSeniorSecuredNotesMember
   
Senior notes, interest rate, percentage 5.00%us-gaap_DebtInstrumentInterestRateEffectivePercentage
/ us-gaap_DebtInstrumentAxis
= cik0000728391_FivePointZeroZeroPercentSeniorSecuredNotesMember
   
Debt instrument, maturity date May 01, 2018    
Utility Segment [Member]      
Debt Instrument [Line Items]      
Net income $ 29,700,000us-gaap_NetIncomeLoss
/ us-gaap_StatementBusinessSegmentsAxis
= cik0000728391_UtilitySegmentMember
$ 31,400,000us-gaap_NetIncomeLoss
/ us-gaap_StatementBusinessSegmentsAxis
= cik0000728391_UtilitySegmentMember
 
Maximum [Member]      
Debt Instrument [Line Items]      
Nonutility assets representation rate, percentage 1.00%cik0000728391_NonutilityAssetsRepresentationRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
  1.00%cik0000728391_NonutilityAssetsRepresentationRatePercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember