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, 2020 USD ($) Boe MMBTU	Dec. 31, 2019 USD ($) Boe MMBTU
Derivative [Line Items]
Derivative assets - Fair value	$ 708,610	$ 330,316
Derivative liabilities - Fair value	$ 506,019	$ 256,528
Customer Counterparty | Crude oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	6,100	7,811
Customer Counterparty | Natural gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	110,340	63,773
Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets - Fair value	$ 708,546	$ 330,316
Derivative liabilities - Fair value	503,906	251,744
Derivatives not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	110,029	81,380
Derivative liabilities - Fair value	118,873	80,517
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	49,589	40,065
Derivative liabilities - Fair value	$ 75,038	$ 42,140
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Written options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	0	36
Derivative assets - Fair value	$ 0	$ 0
Derivative liabilities - Fair value	$ 0	$ 30
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	1,390	3,174
Derivative assets - Fair value	$ 18	$ 2,673
Derivative liabilities - Fair value	$ 12,245	$ 538
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	4,710	4,601
Derivative assets - Fair value	$ 3,635	$ 6,949
Derivative liabilities - Fair value	$ 36,622	$ 5,531
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	6,100	7,811
Derivative assets - Fair value	$ 3,653	$ 9,622
Derivative liabilities - Fair value	$ 48,867	$ 6,099
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Written options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure	973	540
Derivative assets - Fair value	$ 0	$ 0
Derivative liabilities - Fair value	$ 116	$ 22
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	14,761	14,277
Derivative assets - Fair value	$ 3,705	$ 186
Derivative liabilities - Fair value	$ 0	$ 522
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	94,606	48,956
Derivative assets - Fair value	$ 42,231	$ 30,257
Derivative liabilities - Fair value	$ 26,055	$ 35,497
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	110,340	63,773
Derivative assets - Fair value	$ 45,936	$ 30,443
Derivative liabilities - Fair value	26,171	36,041
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	60,440	41,315
Derivative liabilities - Fair value	$ 43,835	$ 38,377
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Purchased options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	0	36
Derivative assets - Fair value	$ 0	$ 29
Derivative liabilities - Fair value	$ 0	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	1,544	3,630
Derivative assets - Fair value	$ 12,946	$ 677
Derivative liabilities - Fair value	$ 943	$ 2,815
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	4,759	4,721
Derivative assets - Fair value	$ 21,336	$ 4,516
Derivative liabilities - Fair value	$ 2,991	$ 5,215
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	6,303	8,387
Derivative assets - Fair value	$ 34,282	$ 5,222
Derivative liabilities - Fair value	$ 3,934	$ 8,030
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Purchased options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure	963	530
Derivative assets - Fair value	$ 115	$ 21
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	18,111	14,517
Derivative assets - Fair value	$ 0	$ 471
Derivative liabilities - Fair value	$ 3,046	$ 150
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	102,263	48,779
Derivative assets - Fair value	$ 26,043	$ 35,601
Derivative liabilities - Fair value	$ 36,855	$ 30,197
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	121,337	63,826
Derivative assets - Fair value	$ 26,158	$ 36,093
Derivative liabilities - Fair value	$ 39,901	$ 30,347