Investment Securities Available for Sale (Narrative) (Details) $ in Thousands	Jun. 30, 2018 USD ($) position	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Financing Receivable, Impaired [Line Items]
Investment securities in a loss position for less than twelve months | position	80
Investment securities in a loss position for twelve months or longer | position	55
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment from adoption of ASU			$ (685)	$ 0
Collateral Pledged
Financing Receivable, Impaired [Line Items]
Pledged to secure deposits	$ 1,350,000		2,000,000
ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment from adoption of ASU	$ 3,200	$ 3,200	$ 0