Income Taxes: Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Components of Deferred Tax Assets [Abstract]
Employee benefits	$ 4,683	$ 4,995
Net operating loss	15	14,794
Regulatory liabilities	9,908	10,824
Other	16,171	2,864
Deferred Tax Assets, Gross	30,777	33,477
Components of Deferred Tax Liabilities [Abstract]
Accelerated depreciation and other plant related differences	(187,666)	(184,478)
Allowance for Funds Used During Construction	(8,571)	(8,365)
Regulatory assets	(4,236)	(3,910)
Employee benefits	(3,003)	(3,723)
Deferred costs	(14,765)	(11,324)
Other	(1,497)	(1,058)
Total deferred tax liabilities	(219,738)	(212,858)
Net deferred tax assets (liabilities)	$ (188,961)	$ (179,381)