BENEFIT PLANS: Level 4 (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Pension Plan
Defined Benefit Plan Disclosure
Service cost	$ 5,120,000	$ 4,417,000	$ 15,359,000	$ 13,253,000
Interest cost	7,581,000	7,279,000	22,742,000	21,839,000
Expected return on plan assets	(7,968,000)	(7,270,000)	(23,903,000)	(19,847,000)
Amortization of transition obligation	0	0	0	0
Amortization of prior service cost	130,000	163,000	389,000	488,000
Amortization of net loss	2,168,000	1,918,000	6,505,000	5,756,000
Net periodic benefit cost	7,031,000	6,507,000	21,092,000	21,489,000
Costs not recognized due to the effects of regulation	(2,371,000)	(4,624,000)	(11,981,000)	(18,650,000)
Net periodic benefit cost recognized for financial reporting	4,660,000	1,883,000	9,111,000	2,839,000
Amount by which the Idaho-jurisdiction pension recovery was increased in the current year	17,100,000		17,100,000
Contributions made to the defined benefit pension plan	18,500,000
Defined Benefit Plan, Minimum Employer Contributions Required in Current Fiscal Year			6,000,000
Senior Management Security Plan
Defined Benefit Plan Disclosure
Service cost	487,000	385,000	1,463,000	1,156,000
Interest cost	773,000	751,000	2,319,000	2,253,000
Expected return on plan assets	0	0	0	0
Amortization of transition obligation	0	0	0	0
Amortization of prior service cost	61,000	59,000	183,000	175,000
Amortization of net loss	323,000	232,000	969,000	698,000
Net periodic benefit cost	1,644,000	1,427,000	4,934,000	4,282,000
Costs not recognized due to the effects of regulation	0	0	0	0
Net periodic benefit cost recognized for financial reporting	1,644,000	1,427,000	4,934,000	4,282,000
Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	330,000	340,000	992,000	1,020,000
Interest cost	859,000	898,000	2,576,000	2,693,000
Expected return on plan assets	(660,000)	(641,000)	(1,981,000)	(1,921,000)
Amortization of transition obligation	510,000	510,000	1,530,000	1,530,000
Amortization of prior service cost	(105,000)	(133,000)	(316,000)	(401,000)
Amortization of net loss	144,000	143,000	433,000	430,000
Net periodic benefit cost	1,078,000	1,117,000	3,234,000	3,351,000
Costs not recognized due to the effects of regulation	0	0	0	0
Net periodic benefit cost recognized for financial reporting	$ 1,078,000	$ 1,117,000	$ 3,234,000	$ 3,351,000