Derivatives (Effects on Consolidated Statements of Operations and Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives, amount of gain (loss) recognized in income		$ 3,802	$ (19,042)	$ 14,925
Embedded Derivative Financial Instruments [Member] | Other Expense
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives, amount of gain (loss) recognized in income	$ (10,472)		0	0
Designated as Hedging Instrument [Member] | Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives, amount of gain (loss) recognized in income				(1,101)
Designated as Hedging Instrument [Member] | Commodity Derivatives [Member] | Onshore facilities and transportation product costs
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives, amount of gain (loss) recognized in income		5,116	(13,195)
Not Designated As Hedging Instrument [Member] | Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives, amount of gain (loss) recognized in income				$ 16,026
Not Designated As Hedging Instrument [Member] | Commodity Derivatives [Member] | Onshore facilities and transportation product costs
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives, amount of gain (loss) recognized in income		$ (1,314)	$ (5,847)