Financial Instruments and Risk Management (Narrative) (Details) $ in Millions	3 Months Ended	9 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($) bbl / d contract item $ / bbl	Sep. 30, 2017 USD ($) bbl / d contract	Dec. 31, 2017 item
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of foreign currency derivatives | contract		0	0
Amount of loss reclassified to interest expense		$ 2.2	$ 2.2
Number of offsetting positions | item		0		0
Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative loss	$ 0.7
Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d		21,000
Average price | $ / bbl		54.88
Interest Rate Swap [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Loss deferred for fair value of interest rate derivative contracts, net of tax		$ 8.5
Income taxes on deferred loss on fair value of interest rate derivative contracts		$ 2.3
Contracts Maturing In 2017 [Member] | Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d			22,000
Contracts Maturing In 2018 [Member] | Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d			6,000
Notes Due 2022 [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Debt instrument term		10 years