Financial Instruments and Risk Management (Carrying Value of Assets and Liabilities Recorded at Fair Value on Recurring Basis) (Details) $ in Thousands	1 Months Ended	9 Months Ended
	Oct. 30, 2019 barrels_per_day $ / bbl	Sep. 30, 2020 bbl / d	Sep. 30, 2019 USD ($) bbl / d $ / bbl	Sep. 30, 2018 barrels_per_day $ / bbl	Jun. 30, 2020 $ / bbl	Dec. 31, 2018 USD ($)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets			$ 104,358			$ 3,837
Liabilities			153,187			61,575
Fair Value, Measurements, Recurring | Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets			104,358			3,837
Fair Value, Measurements, Recurring | Nonqualified employee savings plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities			15,499			13,845
Fair Value, Measurements, Recurring | Contingent consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities			137,688			47,730
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets			0			0
Liabilities			15,499			13,845
Fair Value, Measurements, Recurring | Level 1 | Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets			0			0
Fair Value, Measurements, Recurring | Level 1 | Nonqualified employee savings plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities			15,499			13,845
Fair Value, Measurements, Recurring | Level 1 | Contingent consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities			0			0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets			104,358			3,837
Liabilities			0			0
Fair Value, Measurements, Recurring | Level 2 | Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets			104,358			3,837
Fair Value, Measurements, Recurring | Level 2 | Nonqualified employee savings plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities			0			0
Fair Value, Measurements, Recurring | Level 2 | Contingent consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities			0			0
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets			0			0
Liabilities			137,688			47,730
Fair Value, Measurements, Recurring | Level 3 | Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets			0			0
Fair Value, Measurements, Recurring | Level 3 | Nonqualified employee savings plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities			0			0
Fair Value, Measurements, Recurring | Level 3 | Contingent consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities			$ 137,688			$ 47,730
Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average Forward Price (in dollars per barrel) | $ / bbl			60.51	54.88
Oil and Gas Delivery Commitments and Contracts, Daily Production			35,000	21,000
Subsequent Event | Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average Forward Price (in dollars per barrel) | $ / bbl	56.42				57.59
Oil and Gas Delivery Commitments and Contracts, Daily Production	45,000	35,000