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Investment Securities (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
Integer
Dec. 31, 2016
USD ($)
Integer
Investments, Debt and Equity Securities [Abstract]    
Increase in amortized cost basis of available-for-sale securities $210,000 Net increase to accumulated   $ 210,000
Net increase to accumulated other comprehensive income net of tax   $ 7,400
Number of Investment Securities Sold | Integer 112 84
Available-for-sale Securities Pledged as Collateral $ 157,800 $ 121,900
Description of Holding Securities   there were no holdings of securities of any one issuer, other than the U.S. Government and its agencies, in an amount greater than 10% of stockholders' equity.