DERIVATIVE INSTRUMENTS: (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($) MMBbls item $ / item	Dec. 31, 2020 item
Derivatives designated as hedging instruments under ASC 815
Number of derivative contracts entered | item	2
Derivative instruments held | item		0
Call Option | Natural Gas | Derivatives designated as hedging instruments
Derivatives designated as hedging instruments under ASC 815
Gas volume (MMBTUs) | MMBbls	5,285,000
Fixed price ($) | $ / item	3.75
Estimated fair value of assets (liabilities) as of September 30, 2021 (millions of $)	$ 12.1
(Favorable) unfavorable effect in OCI - net of deferred income taxes (millions of $)	$ (8.4)
Swap contracts | Natural Gas | Derivatives designated as hedging instruments
Derivatives designated as hedging instruments under ASC 815
Gas volume (MMBTUs) | MMBbls	5,285,000
Fixed price ($) | $ / item	0.55
Total option premium (millions of $) | $ / item	2.9
Estimated fair value of assets (liabilities) as of September 30, 2021 (millions of $)	$ (2.9)
(Favorable) unfavorable effect in OCI - net of deferred income taxes (millions of $)	$ 2.0