Derivatives (Derivatives Not Designated as Hedging Instruments - Commodity Contracts) (Details) MMBTU in Thousands, Boe in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021 USD ($) MMBTU Boe	Dec. 31, 2020 USD ($) Boe MMBTU
Derivative [Line Items]
Derivative assets - Fair value	$ 588,582	$ 602,754
Derivative liabilities - Fair value	$ 448,921	$ 424,828
Customer Counterparty | Crude oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	8,484	6,321
Customer Counterparty | Natural gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	119,921	109,635
Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets - Fair value	$ 588,582	$ 602,754
Derivative liabilities - Fair value	447,542	422,729
Derivatives not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	228,536	82,451
Derivative liabilities - Fair value	199,327	84,165
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	203,288	44,762
Derivative liabilities - Fair value	$ 25,224	$ 43,978
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Written options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	210
Derivative assets - Fair value	$ 288
Derivative liabilities - Fair value	$ 2
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	2,406	2,022
Derivative assets - Fair value	$ 28,308	$ 2,344
Derivative liabilities - Fair value	$ 35	$ 2,193
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	5,868	4,299
Derivative assets - Fair value	$ 92,924	$ 9,282
Derivative liabilities - Fair value	$ 27	$ 14,283
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	8,484	6,321
Derivative assets - Fair value	$ 121,520	$ 11,626
Derivative liabilities - Fair value	$ 64	$ 16,476
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Written options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	5,336	597
Derivative assets - Fair value	$ 3	$ 0
Derivative liabilities - Fair value	$ 16	$ 59
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	18,178	12,733
Derivative assets - Fair value	$ 9,327	$ 1,063
Derivative liabilities - Fair value	$ 0	$ 205
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	96,407	96,305
Derivative assets - Fair value	$ 72,438	$ 32,073
Derivative liabilities - Fair value	$ 25,144	$ 27,238
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	119,921	109,635
Derivative assets - Fair value	$ 81,768	$ 33,136
Derivative liabilities - Fair value	25,160	27,502
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	25,248	37,689
Derivative liabilities - Fair value	$ 174,103	$ 40,187
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Purchased options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	210
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair value	$ 241
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	2,406	2,022
Derivative assets - Fair value	$ 35	$ 2,217
Derivative liabilities - Fair value	$ 27,811	$ 2,402
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	5,868	4,299
Derivative assets - Fair value	$ 60	$ 8,220
Derivative liabilities - Fair value	$ 76,813	$ 7,135
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	8,484	6,321
Derivative assets - Fair value	$ 95	$ 10,437
Derivative liabilities - Fair value	$ 104,865	$ 9,537
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Purchased options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	5,336	597
Derivative assets - Fair value	$ 16	$ 59
Derivative liabilities - Fair value	$ 3	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	23,468	16,293
Derivative assets - Fair value	$ 0	$ 205
Derivative liabilities - Fair value	$ 7,879	$ 813
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	103,653	103,973
Derivative assets - Fair value	$ 25,137	$ 26,988
Derivative liabilities - Fair value	$ 61,356	$ 29,837
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	132,457	120,863
Derivative assets - Fair value	$ 25,153	$ 27,252
Derivative liabilities - Fair value	$ 69,238	$ 30,650