Derivative Financial Instruments (Schedule of offsetting financial assets and liabilities for non-customer derivative positions) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Asset, Gross Amount	$ 8,992	$ 35,727
Derivative Asset, Collateral, Obligation to Return Cash, Offset	32,000
Derivative Asset, Net Amount	43	0
Derivative Liability, Gross Amount	2,387	25,300
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	35,100
Derivative Liability, Net Amount	142	446
Subject to and not subject to Master Netting Agreements [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Amount Offset Against Collateral	2,245	24,252
Derivative Asset, Collateral, Obligation to Return Cash, Offset	6,704	11,475
Derivative Asset, Net Amount	23,566	41,910
Derivative Liability, Fair Value, Amount Offset Against Collateral	2,245	24,254
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0	600
Derivative Liability, Net Amount	26,178	20,335
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Gross Amount	2,770	3,270
Derivative Asset, Fair Value, Amount Offset Against Collateral	91	2,335
Derivative Asset, Collateral, Obligation to Return Cash, Offset	2,679	935
Derivative Asset, Net Amount	0	0
Derivative Liability, Gross Amount	0	792
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	792
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0	0
Derivative Liability, Net Amount	0	0
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Gross Amount	6,222	32,457
Derivative Asset, Fair Value, Amount Offset Against Collateral	2,154	21,917
Derivative Asset, Collateral, Obligation to Return Cash, Offset	4,025	10,540
Derivative Asset, Net Amount	43	0
Derivative Liability, Gross Amount	2,387	24,508
Derivative Liability, Fair Value, Amount Offset Against Collateral	2,245	23,462
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0	600
Derivative Liability, Net Amount	$ 142	$ 446