Derivatives (Derivatives Not Designated as Hedging Instruments - Commodity Contracts) (Details) MMBTU in Thousands, Boe in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) MMBTU Boe	Dec. 31, 2020 USD ($) MMBTU Boe
Derivative [Line Items]
Derivative assets - Fair value	$ 694,010	$ 602,754
Derivative liabilities - Fair value	$ 561,788	$ 424,828
Customer Counterparty | Crude oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	8,557	6,321
Customer Counterparty | Natural gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	95,273	109,635
Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets - Fair value	$ 693,852	$ 602,754
Derivative liabilities - Fair value	560,639	422,729
Derivatives not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	363,063	82,451
Derivative liabilities - Fair value	324,027	84,165
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	256,001	44,763
Derivative liabilities - Fair value	$ 14,821	$ 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	212
Derivative assets - Fair value	$ 302
Derivative liabilities - Fair value	$ 1
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	3,070	2,022
Derivative assets - Fair value	$ 35,941	$ 2,344
Derivative liabilities - Fair value	$ 0	$ 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	5,275	4,299
Derivative assets - Fair value	$ 82,563	$ 9,282
Derivative liabilities - Fair value	$ 2	$ 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	8,557	6,321
Derivative assets - Fair value	$ 118,806	$ 11,626
Derivative liabilities - Fair value	$ 3	$ 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	2,668	597
Derivative assets - Fair value	$ 0	$ 0
Derivative liabilities - Fair value	$ 0	$ 59
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	25,224	12,733
Derivative assets - Fair value	$ 30,542	$ 1,063
Derivative liabilities - Fair value	$ 62	$ 205
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	67,381	96,305
Derivative assets - Fair value	$ 106,653	$ 32,074
Derivative liabilities - Fair value	$ 14,756	$ 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	95,273	109,635
Derivative assets - Fair value	$ 137,195	$ 33,137
Derivative liabilities - Fair value	14,818	27,502
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	107,062	37,689
Derivative liabilities - Fair value	$ 309,206	$ 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	212
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair value	$ 278
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	3,126	2,022
Derivative assets - Fair value	$ 0	$ 2,217
Derivative liabilities - Fair value	$ 35,575	$ 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	8,219	4,299
Derivative assets - Fair value	$ 30,604	$ 8,220
Derivative liabilities - Fair value	$ 98,712	$ 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	11,557	6,321
Derivative assets - Fair value	$ 30,604	$ 10,437
Derivative liabilities - Fair value	$ 134,565	$ 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	2,668	597
Derivative assets - Fair value	$ 0	$ 59
Derivative liabilities - Fair value	$ 0	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	29,242	16,293
Derivative assets - Fair value	$ 5,236	$ 205
Derivative liabilities - Fair value	$ 32,005	$ 813
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	127,419	103,973
Derivative assets - Fair value	$ 71,222	$ 26,988
Derivative liabilities - Fair value	$ 142,636	$ 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	159,329	120,863
Derivative assets - Fair value	$ 76,458	$ 27,252
Derivative liabilities - Fair value	$ 174,641	$ 30,650