BENEFIT PLANS: Schedule of Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Reclassification adjustments for:
Unfunded pension liability adjustment, net of tax	$ 13,440	$ (8,120)		$ 5,990
Pension Plan
Defined Benefit Plan Disclosure
Actuarial (loss) gain during the year	(82,631)	(15,226)		(26,608)
Plan amendment service cost	0	0		0
Reclassification adjustments for:
Amortization of net loss	13,564	13,558		13,190
Amortization of prior service cost	6	6		28
Adjustment for deferred tax effects	17,776	428		1,744
Adjustment due to the effects of regulation	51,285	1,234		11,646
Unfunded pension liability adjustment, net of tax	0	0		0
Senior Management Security Plan
Defined Benefit Plan Disclosure
Actuarial (loss) gain during the year	(17,888)	7,049		(10,635)
Plan amendment service cost	(2,839)	0		0
Reclassification adjustments for:
Amortization of net loss	2,533	3,788		2,963
Amortization of prior service cost	96	98		127
Adjustment for deferred tax effects	4,658	(2,815)		1,555
Adjustment due to the effects of regulation	0	0		0
Unfunded pension liability adjustment, net of tax	(13,440)	8,120		(5,990)
Postretirement Benefits
Defined Benefit Plan Disclosure
Actuarial (loss) gain during the year	(249)	(1,109)		(2,964)
Plan amendment service cost	0	0		(212)
Reclassification adjustments for:
Immediate Recognition of loss from temporary deviation	0	4,216	[1],[2]	0
Amortization of prior service cost	48	47		47
Adjustment for deferred tax effects	52	270		807
Adjustment due to the effects of regulation	149	(3,424)		2,322
Unfunded pension liability adjustment, net of tax	$ 0	$ 0		$ 0

[1] In 2018, a loss associated with a temporary deviation from the cost-sharing provisions of the substantive plan was recognized in "Other expense, net" on the consolidated statements of income of the companies.

[2] In 2018, a loss associated with a temporary deviation from the cost-sharing provisions of the substantive plan was recognized in "Other expense, net" on the consolidated statements of income of the companies.