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, 2019 USD ($) Boe MMBTU	Dec. 31, 2018 USD ($) Boe MMBTU
Derivative [Line Items]
Derivative assets - Fair value	$ 358,639	$ 107,816
Derivative liabilities - Fair value	$ 263,734	$ 118,305
Customer Counterparty | Crude oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	7,166	2,507
Customer Counterparty | Natural gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	33,089	14,722
Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets - Fair value	$ 358,639	$ 107,816
Derivative liabilities - Fair value	260,499	111,828
Derivatives not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	32,978	14,422
Derivative liabilities - Fair value	41,867	23,068
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	4,289	1,127
Derivative liabilities - Fair value	$ 36,147	$ 22,687
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Written options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	148	524
Derivative assets - Fair value	$ 0	$ 0
Derivative liabilities - Fair value	$ 305	$ 2,628
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	2,838	872
Derivative assets - Fair value	$ 19	$ 0
Derivative liabilities - Fair value	$ 5,523	$ 3,772
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	4,180	1,111
Derivative assets - Fair value	$ 1,454	$ 0
Derivative liabilities - Fair value	$ 22,806	$ 14,278
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	7,166	2,507
Derivative assets - Fair value	$ 1,473	$ 0
Derivative liabilities - Fair value	$ 28,634	$ 20,678
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Written options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	570
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair value	$ 42
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	10,742	3,063
Derivative assets - Fair value	$ 228	$ 78
Derivative liabilities - Fair value	$ 447	$ 152
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	21,777	11,659
Derivative assets - Fair value	$ 2,588	$ 1,049
Derivative liabilities - Fair value	$ 7,024	$ 1,857
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	33,089	14,722
Derivative assets - Fair value	$ 2,816	$ 1,127
Derivative liabilities - Fair value	7,513	2,009
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	28,689	13,295
Derivative liabilities - Fair value	$ 5,720	$ 381
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Purchased options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	148	524
Derivative assets - Fair value	$ 225	$ 2,251
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	3,384	872
Derivative assets - Fair value	$ 6,141	$ 3,225
Derivative liabilities - Fair value	$ 713	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	4,299	1,111
Derivative assets - Fair value	$ 14,922	$ 5,799
Derivative liabilities - Fair value	$ 1,939	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	7,831	2,507
Derivative assets - Fair value	$ 21,288	$ 11,275
Derivative liabilities - Fair value	$ 2,652	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Purchased options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	560
Derivative assets - Fair value	$ 35
Derivative liabilities - Fair value	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	10,672	3,063
Derivative assets - Fair value	$ 387	$ 151
Derivative liabilities - Fair value	$ 183	$ 64
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	22,938	11,659
Derivative assets - Fair value	$ 6,979	$ 1,869
Derivative liabilities - Fair value	$ 2,885	$ 317
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	34,170	14,722
Derivative assets - Fair value	$ 7,401	$ 2,020
Derivative liabilities - Fair value	$ 3,068	$ 381