Derivatives (Derivatives Not Designated as Hedging Instruments - Commodity Contracts) (Details) MMBTU in Thousands, Boe in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) MMBTU Boe	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Derivative assets - Fair value	$ 107,816	$ 67,077
Derivative liabilities - Fair Value	118,305	75,838
Derivative instruments not designated as hedging instruments
Derivative [Line Items]
Derivative assets - Fair value	107,816	67,077
Derivative liabilities - Fair Value	111,828	69,068
Derivative instruments not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	14,422	0
Derivative liabilities - Fair Value	23,068	$ 0
Commodity contracts | Derivative instruments not designated as hedging instruments | Customer Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	1,127
Derivative liabilities - Fair Value	$ 22,687
Commodity contracts | Derivative instruments not designated as hedging instruments | Customer Counterparty | Crude Oil | Written options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	524
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair Value	$ 2,628
Commodity contracts | Derivative instruments not designated as hedging instruments | Customer Counterparty | Crude Oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	872
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair Value	$ 3,772
Commodity contracts | Derivative instruments not designated as hedging instruments | Customer Counterparty | Crude Oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	1,111
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair Value	$ 14,278
Commodity contracts | Derivative instruments not designated as hedging instruments | Customer Counterparty | Crude Oil | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	2,507
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair Value	$ 20,678
Commodity contracts | Derivative instruments not designated as hedging instruments | Customer Counterparty | Natural Gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	3,063
Derivative assets - Fair value	$ 78
Derivative liabilities - Fair Value	$ 152
Commodity contracts | Derivative instruments not designated as hedging instruments | Customer Counterparty | Natural Gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	11,659
Derivative assets - Fair value	$ 1,049
Derivative liabilities - Fair Value	$ 1,857
Commodity contracts | Derivative instruments not designated as hedging instruments | Customer Counterparty | Natural Gas | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	14,722
Derivative assets - Fair value	$ 1,127
Derivative liabilities - Fair Value	2,009
Commodity contracts | Derivative instruments not designated as hedging instruments | Financial Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	13,295
Derivative liabilities - Fair Value	$ 381
Commodity contracts | Derivative instruments not designated as hedging instruments | Financial Counterparty | Crude Oil | Purchased options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	524
Derivative assets - Fair value	$ 2,251
Derivative liabilities - Fair Value	$ 0
Commodity contracts | Derivative instruments not designated as hedging instruments | Financial Counterparty | Crude Oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	872
Derivative assets - Fair value	$ 3,225
Derivative liabilities - Fair Value	$ 0
Commodity contracts | Derivative instruments not designated as hedging instruments | Financial Counterparty | Crude Oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	1,111
Derivative assets - Fair value	$ 5,799
Derivative liabilities - Fair Value	$ 0
Commodity contracts | Derivative instruments not designated as hedging instruments | Financial Counterparty | Crude Oil | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	2,507
Derivative assets - Fair value	$ 11,275
Derivative liabilities - Fair Value	$ 0
Commodity contracts | Derivative instruments not designated as hedging instruments | Financial Counterparty | Natural Gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	3,063
Derivative assets - Fair value	$ 151
Derivative liabilities - Fair Value	$ 64
Commodity contracts | Derivative instruments not designated as hedging instruments | Financial Counterparty | Natural Gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	11,659
Derivative assets - Fair value	$ 1,869
Derivative liabilities - Fair Value	$ 317
Commodity contracts | Derivative instruments not designated as hedging instruments | Financial Counterparty | Natural Gas | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	14,722
Derivative assets - Fair value	$ 2,020
Derivative liabilities - Fair Value	$ 381