Derivatives (Derivatives Not Designated as Hedging Instruments - Commodity Contracts) (Details) MMBTU in Thousands, Boe in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) Boe MMBTU	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Derivative assets - Fair value	$ 99,103	$ 66,146
Derivative liabilities - Fair value	133,702	74,935
Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets - Fair value	99,042	66,146
Derivative liabilities - Fair value	126,006	68,136
Derivatives not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	12,980	0
Derivative liabilities - Fair value	7,912	0
Derivatives not designated as hedging instruments | Customer Counterparties
Derivative [Line Items]
Derivative assets - Fair value	12,915
Derivative liabilities - Fair value	132
Derivatives not designated as hedging instruments | Customer Counterparties | Written Options
Derivative [Line Items]
Derivative assets - Fair value	0	0
Derivative liabilities - Fair value	782	223
Derivatives not designated as hedging instruments | Customer Counterparties | Collars
Derivative [Line Items]
Derivative assets - Fair value	2
Derivative liabilities - Fair value	0
Derivatives not designated as hedging instruments | Customer Counterparties | Swaps
Derivative [Line Items]
Derivative assets - Fair value	8,520	32,241
Derivative liabilities - Fair value	$ 97,897	24,879
Derivatives not designated as hedging instruments | Customer Counterparties | Crude oil | Written Options
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | Boe	725
Derivative assets - Fair value	$ 842
Derivative liabilities - Fair value	$ 128
Derivatives not designated as hedging instruments | Customer Counterparties | Crude oil | Collars
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | Boe	802
Derivative assets - Fair value	$ 3,363
Derivative liabilities - Fair value	$ 0
Derivatives not designated as hedging instruments | Customer Counterparties | Crude oil | Swaps
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | Boe	1,140
Derivative assets - Fair value	$ 8,128
Derivative liabilities - Fair value	$ 0
Derivatives not designated as hedging instruments | Customer Counterparties | Crude oil | Commodity contracts
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | Boe	2,667
Derivative assets - Fair value	$ 12,333
Derivative liabilities - Fair value	$ 128
Derivatives not designated as hedging instruments | Customer Counterparties | Natural gas | Collars
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | MMBTU	1,360
Derivative assets - Fair value	$ 65
Derivative liabilities - Fair value	$ 0
Derivatives not designated as hedging instruments | Customer Counterparties | Natural gas | Swaps
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | MMBTU	8,500
Derivative assets - Fair value	$ 517
Derivative liabilities - Fair value	$ 4
Derivatives not designated as hedging instruments | Customer Counterparties | Natural gas | Commodity contracts
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | MMBTU	9,860
Derivative assets - Fair value	$ 582
Derivative liabilities - Fair value	4
Derivatives not designated as hedging instruments | Financial Counterparties
Derivative [Line Items]
Derivative assets - Fair value	65
Derivative liabilities - Fair value	7,780
Derivatives not designated as hedging instruments | Financial Counterparties | Purchased Options
Derivative [Line Items]
Derivative assets - Fair value	795	233
Derivative liabilities - Fair value	0	0
Derivatives not designated as hedging instruments | Financial Counterparties | Collars
Derivative [Line Items]
Derivative assets - Fair value	0
Derivative liabilities - Fair value	2
Derivatives not designated as hedging instruments | Financial Counterparties | Swaps
Derivative [Line Items]
Derivative assets - Fair value	61,780	25,684
Derivative liabilities - Fair value	$ 6,768	$ 32,378
Derivatives not designated as hedging instruments | Financial Counterparties | Crude oil | Purchased Options
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | Boe	725
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair value	$ 767
Derivatives not designated as hedging instruments | Financial Counterparties | Crude oil | Collars
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | Boe	802
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair value	$ 3,067
Derivatives not designated as hedging instruments | Financial Counterparties | Crude oil | Swaps
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | Boe	1,140
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair value	$ 3,716
Derivatives not designated as hedging instruments | Financial Counterparties | Crude oil | Commodity contracts
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | Boe	2,667
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair value	$ 7,550
Derivatives not designated as hedging instruments | Financial Counterparties | Natural gas | Collars
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | MMBTU	1,360
Derivative assets - Fair value	$ 0
Derivative liabilities - Fair value	$ 10
Derivatives not designated as hedging instruments | Financial Counterparties | Natural gas | Swaps
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | MMBTU	8,500
Derivative assets - Fair value	$ 65
Derivative liabilities - Fair value	$ 220
Derivatives not designated as hedging instruments | Financial Counterparties | Natural gas | Commodity contracts
Derivative [Line Items]
Notional quantities of commodity contracts in number of barrels of oil and natural gas | MMBTU	9,860
Derivative assets - Fair value	$ 65
Derivative liabilities - Fair value	$ 230