Derivatives (Derivatives Not Designated as Hedging Instruments - Commodity Contracts) (Details) MMBTU in Thousands, Boe in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($) Boe MMBTU	Dec. 31, 2020 USD ($) MMBTU Boe
Derivative [Line Items]
Derivative assets - Fair value	$ 457,317	$ 602,754
Derivative liabilities - Fair value	$ 344,135	$ 424,828
Customer Counterparty | Crude oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	8,258	6,321
Customer Counterparty | Natural gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	102,996	109,635
Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets - Fair value	$ 456,409	$ 602,754
Derivative liabilities - Fair value	342,877	422,729
Derivatives not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	98,429	82,451
Derivative liabilities - Fair value	86,405	84,165
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	78,277	44,762
Derivative liabilities - Fair value	$ 20,218	$ 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	248
Derivative assets - Fair value	$ 211
Derivative liabilities - Fair value	$ 25
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	2,361	2,022
Derivative assets - Fair value	$ 11,181	$ 2,344
Derivative liabilities - Fair value	$ 219	$ 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	5,649	4,299
Derivative assets - Fair value	$ 37,076	$ 9,282
Derivative liabilities - Fair value	$ 1,206	$ 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	8,258	6,321
Derivative assets - Fair value	$ 48,468	$ 11,626
Derivative liabilities - Fair value	$ 1,450	$ 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	8,065	597
Derivative assets - Fair value	$ 52	$ 0
Derivative liabilities - Fair value	$ 319	$ 59
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	10,823	12,733
Derivative assets - Fair value	$ 879	$ 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	84,108	96,305
Derivative assets - Fair value	$ 28,878	$ 32,073
Derivative liabilities - Fair value	$ 18,449	$ 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	102,996	109,635
Derivative assets - Fair value	$ 29,809	$ 33,136
Derivative liabilities - Fair value	18,768	27,502
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	20,152	37,689
Derivative liabilities - Fair value	$ 66,187	$ 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	248
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair value	$ 48
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	2,361	2,022
Derivative assets - Fair value	$ 219	$ 2,217
Derivative liabilities - Fair value	$ 10,989	$ 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	5,649	4,299
Derivative assets - Fair value	$ 1,058	$ 8,220
Derivative liabilities - Fair value	$ 28,624	$ 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	8,258	6,321
Derivative assets - Fair value	$ 1,277	$ 10,437
Derivative liabilities - Fair value	$ 39,661	$ 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	8,065	597
Derivative assets - Fair value	$ 319	$ 59
Derivative liabilities - Fair value	$ 52	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	14,023	16,293
Derivative assets - Fair value	$ 0	$ 205
Derivative liabilities - Fair value	$ 555	$ 813
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	91,790	103,973
Derivative assets - Fair value	$ 18,556	$ 26,988
Derivative liabilities - Fair value	$ 25,919	$ 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	113,878	120,863
Derivative assets - Fair value	$ 18,875	$ 27,252
Derivative liabilities - Fair value	$ 26,526	$ 30,650