Regulatory Accounting: Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Regulatory Liabilities [Line Items]
Total regulatory liabilities	$ 163,793	$ 163,216
Less current regulatory liabilities	(2,079)	(2,574)
Regulatory liabilities, non-current	161,714	160,642
Cost of removal for utility plant
Regulatory Liabilities [Line Items]
Total regulatory liabilities	53,758	52,366
Employee benefit plan costs and related deferred taxes
Regulatory Liabilities [Line Items]
Total regulatory liabilities	7,518	7,518
Excess deferred income taxes
Regulatory Liabilities [Line Items]
Total regulatory liabilities	99,846	100,276
TCJA revenue reserve
Regulatory Liabilities [Line Items]
Total regulatory liabilities	2,079	2,523
Other regulatory liabilities
Regulatory Liabilities [Line Items]
Total regulatory liabilities	$ 592	$ 533