DERIVATIVE FINANCIAL INSTRUMENTS: Level 4 (Details) (USD $)			3 Months Ended				6 Months Ended				3 Months Ended				6 Months Ended				3 Months Ended				6 Months Ended				3 Months Ended				6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Financial swaps Off-system sales		Jun. 30, 2011 Financial swaps Off-system sales		Jun. 30, 2012 Financial swaps Off-system sales		Jun. 30, 2011 Financial swaps Off-system sales		Jun. 30, 2012 Financial swaps Purchased power		Jun. 30, 2011 Financial swaps Purchased power		Jun. 30, 2012 Financial swaps Purchased power		Jun. 30, 2011 Financial swaps Purchased power		Jun. 30, 2012 Financial swaps Fuel Expense		Jun. 30, 2011 Financial swaps Fuel Expense		Jun. 30, 2012 Financial swaps Fuel Expense		Jun. 30, 2011 Financial swaps Fuel Expense		Jun. 30, 2012 Financial swaps Other operating and maintenance		Jun. 30, 2011 Financial swaps Other operating and maintenance		Jun. 30, 2012 Financial swaps Other operating and maintenance		Jun. 30, 2011 Financial swaps Other operating and maintenance		Jun. 30, 2012 Other current assets Financial swaps	Dec. 31, 2011 Other current assets Financial swaps	Jun. 30, 2012 Other current assets Forward contracts	Dec. 31, 2011 Other current assets Forward contracts	Jun. 30, 2012 Other current liabilities Financial swaps	Dec. 31, 2011 Other current liabilities Financial swaps	Jun. 30, 2012 Other current liabilities Forward contracts	Dec. 31, 2011 Other current liabilities Forward contracts	Jun. 30, 2012 Other liabilities Financial swaps	Dec. 31, 2011 Other liabilities Financial swaps	Jun. 30, 2012 Other assets Financial swaps	Dec. 31, 2011 Other assets Financial swaps	Jun. 30, 2012 Other assets Forward contracts	Jun. 30, 2012 Electricity purchases MWh	Jun. 30, 2011 Electricity purchases MWh	Jun. 30, 2012 Electricity sales MWh	Jun. 30, 2011 Electricity sales MWh	Jun. 30, 2012 Natural gas purchases MMBTU	Jun. 30, 2011 Natural gas purchases MMBTU	Jun. 30, 2012 Natural gas sales MMBTU	Jun. 30, 2011 Natural gas sales MMBTU	Jun. 30, 2012 Diesel purchases Gallon	Jun. 30, 2011 Diesel purchases Gallon
Derivative
Derivative Asset, Fair Value, Gross Asset																																			$ 3,262,000	$ 4,361,000	$ 31,000	$ 70,000	$ 3,668,000	$ 1,526,000	$ 134,000		$ 7,000		$ 428,000	$ 359,000	$ 241,000
Derivative Liability, Fair Value, Gross Liability																																			1,946,000	1,036,000			7,043,000	4,755,000	1,898,000	1,370,000	310,000	108,000	312,000
Derivative, Nonmonetary Notional Amount																																																421,550	529,600	1,358,245	764,875	20,295,429	1,908,639	2,238,804	705,622	537,374	449,248
Fair Value
Derivative Asset, Fair Value, Net	7,771,000	6,316,000
Derivative Liability, Fair Value, Net	11,509,000	7,269,000
Derivatives in a net liability position	11,700,000
Collateral Already Posted, Aggregate Fair Value	1,000,000
Additional Collateral, Aggregate Fair Value	7,500,000
Derivative Instruments, Gain (Loss)
Derivative Instruments, Gain Recognized in Income			5,471,000	[1]			9,910,000	[1]	6,506,000	[1]			195,000	[1]			28,000	[1]			386,000	[1]			386,000	[1]	24,000	[1]	227,000	[1]			227,000	[1]
Derivative Instruments, Loss Recognized in Income					$ (215,000)	[1]					$ (2,159,000)	[1]			$ (3,152,000)	[1]			$ (3,633,000)	[1]			$ (3,717,000)	[1]							$ (21,000)	[1]

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