DERIVATIVE FINANCIAL INSTRUMENTS: Level 4 (Details) (USD $)			3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Financial swaps Off-system sales		Sep. 30, 2011 Financial swaps Off-system sales		Sep. 30, 2012 Financial swaps Off-system sales		Sep. 30, 2011 Financial swaps Off-system sales		Sep. 30, 2012 Financial swaps Purchased power		Sep. 30, 2011 Financial swaps Purchased power		Sep. 30, 2012 Financial swaps Purchased power		Sep. 30, 2011 Financial swaps Purchased power		Sep. 30, 2012 Financial swaps Fuel Expense		Sep. 30, 2011 Financial swaps Fuel Expense		Sep. 30, 2012 Financial swaps Fuel Expense		Sep. 30, 2011 Financial swaps Fuel Expense		Sep. 30, 2012 Financial swaps Other operating and maintenance		Sep. 30, 2011 Financial swaps Other operating and maintenance		Sep. 30, 2012 Financial swaps Other operating and maintenance		Sep. 30, 2011 Financial swaps Other operating and maintenance		Sep. 30, 2012 Forward contracts Fuel Expense		Sep. 30, 2011 Forward contracts Fuel Expense		Sep. 30, 2012 Forward contracts Fuel Expense		Sep. 30, 2011 Forward contracts Fuel Expense		Sep. 30, 2012 Other current assets Financial swaps	Dec. 31, 2011 Other current assets Financial swaps	Sep. 30, 2012 Other current assets Forward contracts	Dec. 31, 2011 Other current assets Forward contracts	Sep. 30, 2012 Other current liabilities Financial swaps	Dec. 31, 2011 Other current liabilities Financial swaps	Dec. 31, 2011 Other current liabilities Forward contracts	Sep. 30, 2012 Other liabilities Financial swaps	Dec. 31, 2011 Other liabilities Financial swaps	Sep. 30, 2012 Other assets Financial swaps	Dec. 31, 2011 Other assets Financial swaps	Sep. 30, 2012 Other assets Forward contracts	Sep. 30, 2012 Electricity purchases MWh	Sep. 30, 2011 Electricity purchases MWh	Sep. 30, 2012 Electricity sales MWh	Sep. 30, 2011 Electricity sales MWh	Sep. 30, 2012 Natural gas purchases MMBTU	Sep. 30, 2011 Natural gas purchases MMBTU	Sep. 30, 2012 Natural gas sales MMBTU	Sep. 30, 2011 Natural gas sales MMBTU	Sep. 30, 2012 Diesel purchases Gallon	Sep. 30, 2011 Diesel purchases Gallon
Derivative
Derivative Asset, Fair Value, Gross Asset																																											$ 5,885,000	$ 4,361,000	$ 183,000	$ 70,000	$ 1,930,000	$ 1,526,000		$ 171,000		$ 213,000	$ 359,000	$ 188,000
Derivative Liability, Fair Value, Gross Liability																																											1,781,000	1,036,000			4,582,000	4,755,000	1,370,000	314,000	108,000	115,000
Derivative, Nonmonetary Notional Amount																																																							339,790	197,800	1,691,650	1,038,095	16,691,024	2,292,738	2,911,039	77,500	263,167	266,375
Fair Value
Derivative Asset, Fair Value, Net	8,570,000	6,316,000
Derivative Liability, Fair Value, Net	6,792,000	7,269,000
Derivatives in a net liability position	7,200,000
Collateral Already Posted, Aggregate Fair Value	1,200,000
Additional Collateral, Aggregate Fair Value	4,400,000
Derivative Instruments, Gain (Loss)
Derivative Instruments, Gain Recognized in Income			1,793,000	[1]	441,000	[1]	11,703,000	[1]	6,947,000	[1]											115,000	[1]			501,000	[1]			120,000	[1]			347,000	[1]
Derivative Instruments, Loss Recognized in Income											$ (2,479,000)	[1]	$ (6,982,000)	[1]	$ (5,631,000)	[1]	$ (6,954,000)	[1]	$ (2,516,000)	[1]			$ (6,233,000)	[1]			$ (145,000)	[1]			$ (166,000)	[1]			$ (1,778,000)	[1]	$ 0	[1]	$ (1,778,000)	[1]	$ 0	[1]

[1] Excludes changes in fair value of derivatives, which are recorded on the balance sheet as regulatory assets or regulatory liabilities.