BENEFIT PLANS: Schedule Defined Benefit Plans Disclosures (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Change in Fair Value of Plan Assets [Roll Forward]
Pension amounts recognized in noncurrent liabilities	$ (394,030,000)	$ (403,334,000)
Regulatory Assets	1,354,425,000	1,242,387,000
Pension Plan
Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning balance	844,812,000	695,093,000
Service cost	33,164,000	25,292,000	$ 31,357,000
Interest cost	35,171,000	35,415,000	31,830,000
Actuarial (gain) loss	(47,952,000)	114,496,000
Benefits paid	(29,672,000)	(25,484,000)
Benefit obligation, ending balance	835,523,000	844,812,000	695,093,000
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning balance	559,719,000	545,092,000
Actual return on plan assets	(9,431,000)	10,111,000
Employer contributions	39,000,000	30,000,000
Benefits paid	(29,672,000)	(25,484,000)
Fair value of plan assets, ending balance	559,616,000	559,719,000	545,092,000
Funded status at end of year	(275,907,000)	(285,093,000)
Pension amounts recognized in other current liabilities	0	0
Pension amounts recognized in noncurrent liabilities	(275,907,000)	(285,093,000)
Pension amounts recognized in statement of financial position	(275,907,000)	(285,093,000)
Amounts recognized in accumulated other comprehensive income - net loss	253,212,000	263,350,000
Amounts recognized in accumulated other comprehensive income - prior service cost	74,000	295,000
Amounts recognized in accumulated other comprehensive income - before regulatory asset	253,286,000	263,645,000
Regulatory Assets	253,286,000	263,645,000
Other comprehensive income recognized related to pension benefit plans	0	0
Accumulated benefit obligation	714,994,000	719,617,000
Senior Management Security Plan
Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning balance	94,410,000	77,773,000
Service cost	1,689,000	1,645,000	2,178,000
Interest cost	3,868,000	3,856,000	3,258,000
Actuarial (gain) loss	(352,000)	15,324,000
Benefits paid	(4,226,000)	(4,188,000)
Benefit obligation, ending balance	95,389,000	94,410,000	$ 77,773,000
Change in Fair Value of Plan Assets [Roll Forward]
Benefits paid	(4,226,000)	(4,188,000)
Fair value of plan assets, ending balance	0
Funded status at end of year	(95,389,000)	(94,410,000)
Pension amounts recognized in other current liabilities	(4,423,000)	(4,193,000)
Pension amounts recognized in noncurrent liabilities	(90,966,000)	(90,217,000)
Pension amounts recognized in statement of financial position	(95,389,000)	(94,410,000)
Amounts recognized in accumulated other comprehensive income - net loss	34,260,000	38,808,000
Amounts recognized in accumulated other comprehensive income - prior service cost	673,000	857,000
Amounts recognized in accumulated other comprehensive income - before regulatory asset	34,933,000	39,665,000
Regulatory Assets	0	0
Other comprehensive income recognized related to pension benefit plans	34,933,000	39,665,000
Accumulated benefit obligation	$ 86,838,000	$ 84,684,000