Regulatory Assets and Liabilities (Details) (USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011	Dec. 31, 2010	Mar. 15, 2010	Feb. 12, 2010
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Total regulatory assets and liabilities, net	$ 433,314,000	$ 433,314,000	$ 406,108,000
Deferred construction carrying costs [Abstract]
Percentage of jurisdictional portion of construction carrying costs authorized to be collected from customers (in hundredths)		75.00%
Maximum percentage of projected retail revenues collected from customers (in hundredths)		6.50%
Amount of construction carrying costs to be returned to customers					183,200,000
Period over which construction carrying costs would be returned to customers (in years)		five
Revised period over which construction carrying costs would be returned to customers (in years)		four
Revised amount of construction carrying costs to be returned to customers				167,000,000
Amount of construction carrying costs returned to customers	101,200,000	101,200,000
Amount of construction carrying costs to be returned to customers within one year	38,800,000	38,800,000
Deferred fuel and purchased power costs [Abstract]
Percentage of total fuel cost regulated by LPSC (in hundredths)	95.00%
Change in under-recovered fuel costs		10,600,000
Deferral of additional fuel and purchased power costs		17,600,000
Change in mark-to-market losses on gas positions		7,400,000
Change in deferred gains (losses)		(500,000)
Regulatory assets and liabilities - deferred taxes, net [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Total regulatory assets and liabilities, net	208,247,000	208,247,000	203,696,000
Deferred mining costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	20,392,000	20,392,000	21,666,000
Deferred interest costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	6,850,000	6,850,000	7,033,000
Deferred asset removal costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	799,000	799,000	768,000
Deferred postretirement plan costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	114,909,000	114,909,000	117,651,000
Deferred tree trimming costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	9,729,000	9,729,000	11,086,000
Deferred training costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	7,564,000	7,564,000	7,642,000
Deferred storm surcredits, net [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	11,138,000	11,138,000	10,633,000
Deferred construction carrying costs - assets [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	14,202,000	14,202,000	18,830,000
Lignite mining agreement contingency [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	3,781,000	3,781,000	3,781,000
AFUDC equity gross-up [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	74,753,000	74,753,000	74,859,000
Deferred rate case costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	1,385,000	1,385,000	1,654,000
Deferred Acadia Unit 1 acquisition costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	3,024,000	3,024,000	3,076,000
Deferred IRP/RFP costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	742,000	742,000	977,000
Deferred AMI pilot costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	218,000	218,000	283,000
Total regulatory assets - other [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	269,486,000	269,486,000	279,939,000
Deferred construction carrying costs - liability [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory liabilities	65,405,000	65,405,000	87,875,000
Deferred fuel and purchased power [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	$ 20,986,000	$ 20,986,000	$ 10,348,000