Regulatory Assets and Liabilities (Details) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 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	$ 444,990,000	$ 444,990,000	$ 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				166,400,000
Amount of construction carrying costs returned to customers	126,100,000	126,100,000
Amount of construction carrying costs to be returned to customers within one year	33,300,000	33,300,000
Deferred fuel and purchased power costs [Abstract]
Percentage of total fuel cost regulated by LPSC (in hundredths)	94.00%
Change in under-recovered fuel costs		(1,200,000)
Deferral of additional fuel and purchased power costs		8,500,000
Change in mark-to-market losses on gas positions		(9,500,000)
Regulatory assets and liabilities - deferred taxes, net [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Total regulatory assets and liabilities, net	213,847,000	213,847,000	203,696,000
Deferred mining costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	19,755,000	19,755,000	21,666,000
Deferred interest costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	6,758,000	6,758,000	7,033,000
Deferred asset removal costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	814,000	814,000	768,000
Deferred postretirement plan costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	113,538,000	113,538,000	117,651,000
Deferred tree trimming costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	9,057,000	9,057,000	11,086,000
Deferred training costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	7,525,000	7,525,000	7,642,000
Deferred storm surcredits, net [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	9,080,000	9,080,000	10,633,000
Deferred construction carrying costs - assets [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	12,416,000	12,416,000	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	74,496,000	74,496,000	74,859,000
Deferred rate case costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	1,252,000	1,252,000	1,654,000
Deferred Acadia Unit 1 acquisition costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	2,997,000	2,997,000	3,076,000
Deferred IRP/RFP costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	625,000	625,000	977,000
Deferred AMI pilot costs [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	185,000	185,000	283,000
Total regulatory assets - other [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	262,279,000	262,279,000	279,939,000
Deferred construction carrying costs - liability [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory Liabilities	40,298,000	40,298,000	87,875,000
Deferred fuel and purchased power [Member]
Schedule of Regulatory Assets and Liabilities, Net [Line Items]
Regulatory assets	$ 9,162,000	$ 9,162,000	$ 10,348,000