Financial Instruments and Risk Management (Carrying Value of Assets and Liabilities Recorded at Fair Value on Recurring Basis) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 bbl / d $ / bbl	Jun. 30, 2019 USD ($) bbl / d $ / bbl	Jun. 30, 2018 barrels_per_day	Dec. 31, 2018 USD ($)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		$ 56,193		$ 3,837
Liabilities		194,470		61,575
Fair Value, Measurements, Recurring | Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets		56,193		3,837
Fair Value, Measurements, Recurring | Nonqualified employee savings plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities		16,061		13,845
Fair Value, Measurements, Recurring | Contingent consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities		178,409		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		16,061		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		16,061		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		56,193		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		56,193		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		178,409		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		$ 178,409		$ 47,730
Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, Average Forward Price | $ / bbl		63.64
Oil and Gas Delivery Commitments and Contracts, Daily Production		20,000	21,000
Subsequent Event | Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, Average Forward Price | $ / bbl	60.10
Oil and Gas Delivery Commitments and Contracts, Daily Production | bbl / d	20,000