Derivative Financial Instruments (Counterparty Credit Risk) (Narrative) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Derivative, Collateral, Obligation to Return Cash	$ 406
Net margin collateral posted	(3,100)
Initial margin	32,000
Variation margin collateral received	35,100
Maximum exposure	23,000
Market Approach Valuation Technique [Member]
Derivative [Line Items]
Maximum exposure	$ 28,200