BENEFIT PLANS: Schedule of Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification adjustments for:
Unfunded pension liability adjustment (net of tax)	$ (2,882)	$ 7,605	$ (4,699)
Pension Plan
Defined Benefit Plan Disclosure
Actuarial (loss) gain during the year	(3,790)	(146,674)	154,261
Reclassification adjustments for:
Amortization of net loss	13,927	3,911	17,118
Amortization of prior service cost	221	347	347
Adjustment for deferred tax effects	(4,050)	55,678	(67,136)
Adjustment due to the effects of regulation	(6,308)	86,738	(104,590)
Unfunded pension liability adjustment (net of tax)	0	0	0
Senior Management Security Plan
Defined Benefit Plan Disclosure
Actuarial (loss) gain during the year	353	(15,324)	4,664
Reclassification adjustments for:
Amortization of net loss	4,195	2,618	2,840
Amortization of prior service cost	185	220	212
Adjustment for deferred tax effects	(1,851)	4,881	(3,017)
Unfunded pension liability adjustment (net of tax)	2,882	(7,605)	4,699
Postretirement Benefits
Defined Benefit Plan Disclosure
Actuarial (loss) gain during the year	2,413	(5,733)	20,673
Reclassification adjustments for:
Amortization of net loss	0	0	98
Amortization of prior service cost	15	183	(229)
Adjustment for deferred tax effects	(949)	2,170	(8,031)
Adjustment due to the effects of regulation	(1,479)	3,380	(12,511)
Unfunded pension liability adjustment (net of tax)	$ 0	$ 0	$ 0