Derivatives (Commodity Derivative Positions) (Details) MMBTU in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) MMBTU Boe	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)
Derivative [Line Items]
Derivative assets	$ 81,456,000	$ 66,146,000
Derivative liabilities	114,542,000	74,935,000
Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets	79,140,000	66,146,000
Derivative liabilities	105,895,000	68,136,000
Derivatives not designated as hedging instruments | Commodity swaps and options
Derivative [Line Items]
Derivative assets	3,628,000	0
Derivative liabilities	3,159,000	$ 0
Customer Counterparties | Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets	3,628
Derivative liabilities	271
Customer Counterparties | Derivatives not designated as hedging instruments | Written Option
Derivative [Line Items]
Derivative assets	0		$ 0
Derivative liabilities	696,000		223,000
Customer Counterparties | Derivatives not designated as hedging instruments | Swaps
Derivative [Line Items]
Derivative assets	15,902,000		32,241,000
Derivative liabilities	$ 76,297,000		24,879,000
Customer Counterparties | Derivatives not designated as hedging instruments | Crude oil | Written Option
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | Boe	736,500
Derivative assets	$ 576
Derivative liabilities	$ 271
Customer Counterparties | Derivatives not designated as hedging instruments | Crude oil | Collars
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | Boe	812,100
Derivative assets	$ 1,720
Derivative liabilities	$ 0
Customer Counterparties | Derivatives not designated as hedging instruments | Crude oil | Swaps
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | Boe	292,811
Derivative assets	$ 925
Derivative liabilities	$ 0
Customer Counterparties | Derivatives not designated as hedging instruments | Crude oil | Commodity swaps and options
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | Boe	1,841,411
Derivative assets	$ 3,221
Derivative liabilities	$ 271
Customer Counterparties | Derivatives not designated as hedging instruments | Natural gas | Collars
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | MMBTU	1,540
Derivative assets	$ 85
Derivative liabilities	$ 0
Customer Counterparties | Derivatives not designated as hedging instruments | Natural gas | Swaps
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | MMBTU	3,650
Derivative assets	$ 322
Derivative liabilities	$ 0
Customer Counterparties | Derivatives not designated as hedging instruments | Natural gas | Commodity swaps and options
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | MMBTU	5,190
Derivative assets	$ 407
Derivative liabilities	0
Financial Counterparties | Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets	0
Derivative liabilities	2,888
Financial Counterparties | Derivatives not designated as hedging instruments | Purchased Options
Derivative [Line Items]
Derivative assets	711,000		233,000
Derivative liabilities	0		0
Financial Counterparties | Derivatives not designated as hedging instruments | Swaps
Derivative [Line Items]
Derivative assets	46,450,000		25,684,000
Derivative liabilities	$ 13,234,000		$ 32,378,000
Financial Counterparties | Derivatives not designated as hedging instruments | Crude oil | Purchased Options
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | Boe	736,500
Derivative assets	$ 0
Derivative liabilities	$ 510
Financial Counterparties | Derivatives not designated as hedging instruments | Crude oil | Collars
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | Boe	812,100
Derivative assets	$ 0
Derivative liabilities	$ 1,499
Financial Counterparties | Derivatives not designated as hedging instruments | Crude oil | Swaps
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | Boe	292,811
Derivative assets	$ 0
Derivative liabilities	$ 863
Financial Counterparties | Derivatives not designated as hedging instruments | Crude oil | Commodity swaps and options
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | Boe	1,841,411
Derivative assets	$ 0
Derivative liabilities	$ 2,872
Financial Counterparties | Derivatives not designated as hedging instruments | Natural gas | Collars
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | MMBTU	1,540
Derivative assets	$ 0
Derivative liabilities	$ 4
Financial Counterparties | Derivatives not designated as hedging instruments | Natural gas | Swaps
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | MMBTU	3,650
Derivative assets	$ 0
Derivative liabilities	$ 12
Financial Counterparties | Derivatives not designated as hedging instruments | Natural gas | Commodity swaps and options
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil and gas | MMBTU	5,190
Derivative assets	$ 0
Derivative liabilities	$ 16