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Investment Securities - Credit loss rollforward (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]    
Beginning balance of credit losses recognized $ 874 $ 747
Amounts related to credit loss for which an OTTI charge was not previously recognized 0 136
Realized losses for securities sold 0 (9)
Ending balance of credit losses recognized $ 874 $ 874