Risk Management Activities: Utilities (Details) - Natural Gas, Distribution $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) MMBTU	Mar. 31, 2018 MMBTU	Dec. 31, 2018 MMBTU
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months | $	$ 0.1
Cash Flow Hedging
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure	534,000
Natural gas futures purchased
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	3,120,000	6,760,000	4,000,000
Derivative, Remaining Maturity	21 months	33 months	24 months
Natural gas options purchased, net
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	1,150,000	170,000	4,320,000
Derivative, Remaining Maturity	10 months	11 months	13 months
Natural gas basis swaps purchased
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	3,020,000	6,770,000	3,960,000
Derivative, Remaining Maturity	21 months	33 months	24 months
Natural gas over-the-counter swaps, net
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	3,316,000	2,760,000	3,660,000
Derivative, Remaining Maturity	26 months	26 months	24 months
Credit Risk Derivative Liabilities, at Fair Value | $	$ 0.2
Natural gas physical contracts, net
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	2,786,980	386,250	18,325,852
Derivative, Remaining Maturity	12 months	32 months	30 months