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, 2020 USD ($) MMBTU Boe	Jun. 30, 2019 MMBTU Boe	Dec. 31, 2019 USD ($) Boe MMBTU
Derivative [Line Items]
Derivative assets - Fair value	$ 764,430		$ 330,316
Derivative liabilities - Fair value	$ 560,169		$ 256,528
Customer Counterparty | Crude oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	6,155		7,811
Customer Counterparty | Natural gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	84,957		63,773
Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets - Fair value	$ 764,185		$ 330,316
Derivative liabilities - Fair value	558,512		251,744
Derivatives not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	117,447		81,380
Derivative liabilities - Fair value	138,298		80,517
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	31,767		40,065
Derivative liabilities - Fair value	$ 104,968		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	$ 70		30
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	1,966	3,174
Derivative assets - Fair value	$ 63		2,673
Derivative liabilities - Fair value	$ 20,463		538
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	4,189	4,601
Derivative assets - Fair value	$ 1,273		6,949
Derivative liabilities - Fair value	$ 51,320		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	6,155	7,811
Derivative assets - Fair value	$ 1,336		9,622
Derivative liabilities - Fair value	$ 71,853		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	1,033	540
Derivative assets - Fair value	$ 0		0
Derivative liabilities - Fair value	$ 172		22
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	13,066	14,277
Derivative assets - Fair value	$ 881		186
Derivative liabilities - Fair value	$ 955		522
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	70,858	48,956
Derivative assets - Fair value	$ 29,550		30,257
Derivative liabilities - Fair value	$ 31,988		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	84,957	63,773
Derivative assets - Fair value	$ 30,431		30,443
Derivative liabilities - Fair value	33,115		36,041
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	85,680		41,315
Derivative liabilities - Fair value	$ 33,330		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	$ 70		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	2,218	3,630
Derivative assets - Fair value	$ 21,847		677
Derivative liabilities - Fair value	$ 1,607		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	4,261	4,721
Derivative assets - Fair value	$ 32,875		4,516
Derivative liabilities - Fair value	$ 1,779		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	6,479	8,387
Derivative assets - Fair value	$ 54,792		5,222
Derivative liabilities - Fair value	$ 3,386		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	1,023	530
Derivative assets - Fair value	$ 157		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	13,186	14,517
Derivative assets - Fair value	$ 883		471
Derivative liabilities - Fair value	$ 881		150
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	78,528	48,779
Derivative assets - Fair value	$ 29,848		35,601
Derivative liabilities - Fair value	$ 29,063		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	92,737	63,826
Derivative assets - Fair value	$ 30,888		36,093
Derivative liabilities - Fair value	$ 29,944		$ 30,347