Derivatives (Fair Values of Derivative Contracts) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross fair value of assets	$ 30,422	$ 4,416
Gross liabilities offset against assets in the Balance Sheet	(11,052)	(4,192)
Net fair value of assets presented in the Balance Sheet	19,370	224
Gross fair value of liabilities	12,942	24,609
Gross assets offset against liabilities in the Balance Sheet	(11,052)	(4,192)
Net fair value of liabilities presented in the Balance Sheet	1,890	20,417
Derivatives, current assets
Derivatives, Fair Value [Line Items]
Gross fair value of assets	22,512	1,406
Gross liabilities offset against assets in the Balance Sheet	(6,089)	(1,190)
Net fair value of assets presented in the Balance Sheet	16,423	216
Derivatives, long-term assets
Derivatives, Fair Value [Line Items]
Gross fair value of assets	7,910	3,010
Gross liabilities offset against assets in the Balance Sheet	(4,963)	(3,002)
Net fair value of assets presented in the Balance Sheet	2,947	8
Derivatives, current liabilities
Derivatives, Fair Value [Line Items]
Gross fair value of liabilities	7,799	20,493
Gross assets offset against liabilities in the Balance Sheet	(6,089)	(1,190)
Net fair value of liabilities presented in the Balance Sheet	1,710	19,303
Derivatives, long-term liabilities
Derivatives, Fair Value [Line Items]
Gross fair value of liabilities	5,143	4,116
Gross assets offset against liabilities in the Balance Sheet	(4,963)	(3,002)
Net fair value of liabilities presented in the Balance Sheet	$ 180	$ 1,114

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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