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, 2020 USD ($) MMBTU Boe	Mar. 31, 2019 MMBTU Boe	Dec. 31, 2019 USD ($) MMBTU Boe
Derivative [Line Items]
Derivative assets - Fair value	$ 834,204		$ 330,316
Derivative liabilities - Fair value	$ 658,515		$ 256,528
Customer Counterparty | Crude oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	6,738		7,811
Customer Counterparty | Natural gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	61,296		63,773
Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets - Fair value	$ 834,131		$ 330,316
Derivative liabilities - Fair value	657,340		251,744
Derivatives not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	163,563		81,380
Derivative liabilities - Fair value	199,288		80,517
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	27,134		40,065
Derivative liabilities - Fair value	$ 167,127		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	24	36
Derivative assets - Fair value	$ 0		0
Derivative liabilities - Fair value	$ 681		30
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	2,522	3,174
Derivative assets - Fair value	$ 13		2,673
Derivative liabilities - Fair value	$ 46,210		538
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	4,192	4,601
Derivative assets - Fair value	$ 789		6,949
Derivative liabilities - Fair value	$ 87,097		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	6,738	7,811
Derivative assets - Fair value	$ 802		9,622
Derivative liabilities - Fair value	$ 133,988		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	420	540
Derivative assets - Fair value	$ 0		0
Derivative liabilities - Fair value	$ 43		22
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	14,201	14,277
Derivative assets - Fair value	$ 541		186
Derivative liabilities - Fair value	$ 1,364		522
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	46,675	48,956
Derivative assets - Fair value	$ 25,791		30,257
Derivative liabilities - Fair value	$ 31,732		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	61,296	63,773
Derivative assets - Fair value	$ 26,332		30,443
Derivative liabilities - Fair value	33,139		36,041
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	136,429		41,315
Derivative liabilities - Fair value	$ 32,161		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	24	36
Derivative assets - Fair value	$ 681		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	2,859	3,630
Derivative assets - Fair value	$ 49,546		677
Derivative liabilities - Fair value	$ 3,732		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	4,294	4,721
Derivative assets - Fair value	$ 54,366		4,516
Derivative liabilities - Fair value	$ 2,408		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	7,177	8,387
Derivative assets - Fair value	$ 104,593		5,222
Derivative liabilities - Fair value	$ 6,140		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	410	530
Derivative assets - Fair value	$ 22		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	14,291	14,517
Derivative assets - Fair value	$ 996		471
Derivative liabilities - Fair value	$ 403		150
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	46,500	48,779
Derivative assets - Fair value	$ 30,818		35,601
Derivative liabilities - Fair value	$ 25,618		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	61,201	63,826
Derivative assets - Fair value	$ 31,836		36,093
Derivative liabilities - Fair value	$ 26,021		$ 30,347