Financial Instruments and Risk Management (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) OffsettingPosition contract	Mar. 31, 2018 USD ($) contract	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($) bbl / d OffsettingPosition contract	Jun. 30, 2018 barrels_per_day	Dec. 31, 2018 item
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of foreign currency derivatives | contract	0	0		0
Amount of loss reclassified to interest expense	$ 1,500,000	$ 700,000
Number of Offsetting Positions (in positions)	0			0		0
Scenario, Forecast
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative loss			$ 1,500,000
Commodity derivative contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day)				20,000	21,000
Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Loss deferred for fair value of interest rate derivative contracts, net of tax	$ 6,700,000			$ 6,700,000
Income taxes on deferred loss on fair value of interest rate derivative contracts	$ 1,800,000			$ 1,800,000
Notes Due 2022
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Debt instrument term				10 years