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Loans and Allowance for Credit Losses Allowance for Credit Losses (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Receivables [Abstract]        
Allowance for loan losses $ 169,910 $ 168,679 $ 169,054  
Reserve for unfunded lending commitments 6,174 2,646 2,358  
Allowance for credit losses $ 176,084 $ 171,325 $ 171,412 $ 185,931