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Term Debt (Narrative) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Debt Disclosure [Abstract]    
Term debt $ 802,357 $ 852,397
Maximum outstanding FHLB advances at any month end 850,100 900,200
Average outstanding FHLB advances $ 844,400 $ 894,200
Weighted Average    
Debt Instrument [Line Items]    
Average contractual interest rate on borrowings 1.70% 1.70%