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, 2019 USD ($) MMBTU Boe	Dec. 31, 2018 USD ($) MMBTU Boe
Derivative [Line Items]
Derivative assets - Fair value	$ 242,876	$ 107,816
Derivative liabilities - Fair value	$ 172,923	$ 118,305
Crude oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	5,646	2,507
Natural gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	30,775	14,722
Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets - Fair value	$ 242,824	$ 107,816
Derivative liabilities - Fair value	169,904	111,828
Derivatives not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	22,651	14,422
Derivative liabilities - Fair value	25,906	23,068
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	6,004	1,127
Derivative liabilities - Fair value	$ 18,802	$ 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	134	524
Derivative assets - Fair value	$ 231	$ 0
Derivative liabilities - Fair value	$ 286	$ 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	2,082	872
Derivative assets - Fair value	$ 752	$ 0
Derivative liabilities - Fair value	$ 1,506	$ 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	3,430	1,111
Derivative assets - Fair value	$ 1,985	$ 0
Derivative liabilities - Fair value	$ 10,167	$ 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	5,646	2,507
Derivative assets - Fair value	$ 2,968	$ 0
Derivative liabilities - Fair value	$ 11,959	$ 20,678
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	6,385	3,063
Derivative assets - Fair value	$ 9	$ 78
Derivative liabilities - Fair value	$ 431	$ 152
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	24,390	11,659
Derivative assets - Fair value	$ 3,027	$ 1,049
Derivative liabilities - Fair value	$ 6,412	$ 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	30,775	14,722
Derivative assets - Fair value	$ 3,036	$ 1,127
Derivative liabilities - Fair value	6,843	2,009
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	16,647	13,295
Derivative liabilities - Fair value	$ 7,104	$ 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	134	524
Derivative assets - Fair value	$ 189	$ 2,251
Derivative liabilities - Fair value	$ 222	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	2,697	872
Derivative assets - Fair value	$ 1,752	$ 3,225
Derivative liabilities - Fair value	$ 1,069	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	3,551	1,111
Derivative assets - Fair value	$ 7,622	$ 5,799
Derivative liabilities - Fair value	$ 2,180	$ 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	6,382	2,507
Derivative assets - Fair value	$ 9,563	$ 11,275
Derivative liabilities - Fair value	$ 3,471	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	6,315	3,063
Derivative assets - Fair value	$ 372	$ 151
Derivative liabilities - Fair value	$ 9	$ 64
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	26,807	11,659
Derivative assets - Fair value	$ 6,712	$ 1,869
Derivative liabilities - Fair value	$ 3,624	$ 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	33,122	14,722
Derivative assets - Fair value	$ 7,084	$ 2,020
Derivative liabilities - Fair value	$ 3,633	$ 381