Financial Instruments and Risk Management (Narrative) (Details) barrels_per_day in Thousands, $ in Millions	3 Months Ended		9 Months Ended		24 Months Ended
	Dec. 31, 2020 barrels_per_day $ / bbl	Dec. 31, 2019 barrels_per_day OffsettingPosition	Sep. 30, 2020 USD ($) barrels_per_day OffsettingPosition contract $ / bbl	Sep. 30, 2019 USD ($) barrels_per_day contract $ / bbl	Dec. 31, 2019 Acquisition OffsettingPosition	Dec. 31, 2021 $ / bbl
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of foreign currency derivatives (in contracts) | contract			0	0
Amount of loss reclassified to interest expense			$ 1.1	$ 2.2
Income taxes on deferred loss on fair value of interest rate derivative contracts			2.0
Derivative loss			$ 0.4
Number of businesses acquired (in acquisitions) | Acquisition					2
Number of offsetting positions (in offsetting positions) | OffsettingPosition		0	0		0
Commodity derivative contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | barrels_per_day		35	45	35
Average forward price (in dollars per barrel) | $ / bbl			56.42	60.51
Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Income taxes on deferred loss on fair value of interest rate derivative contracts			$ 0.5
Notes Due 2022
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Debt instrument term			10 years
Forecast | Commodity derivative contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Average forward price (in dollars per barrel) | $ / bbl	57.59					43.31
Forecast | Commodity derivative contracts | Subsequent Event
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | barrels_per_day	18