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Debt Securities Available-for-Sale - Narrative (Details)
12 Months Ended
Dec. 31, 2019
USD ($)
security
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt Securities, Available-for-sale [Line Items]      
Carrying value of securities available-for-sale | $ $ 730,900,000 $ 492,400,000  
Gross proceeds on sales of securities | $ 79,255,000 32,115,000 $ 4,975,000
Gross realized gains | $ 514,000 183,000 173,000
Gross realized losses | $ 0 5,000 4,000
Net gains (losses) on trading securities portfolio | $ 2,000,000.0 (879,000) 1,100,000
Other-than-temporary impairment charges on debt securities available-for-sale | $ $ 0 $ 0 $ 0
Non-GSE | REMIC      
Debt Securities, Available-for-sale [Line Items]      
Number of debt securities, unrealized loss position longer than 12 months 1    
Corporate bonds | REMIC      
Debt Securities, Available-for-sale [Line Items]      
Number of debt securities, unrealized loss position longer than 12 months 4    
Pass-through | GSE | REMIC      
Debt Securities, Available-for-sale [Line Items]      
Number of debt securities, unrealized loss position longer than 12 months 28    
Pass-through | GSE | Pass-through certificates      
Debt Securities, Available-for-sale [Line Items]      
Number of debt securities, unrealized loss position less than 12 months 3    
Non Pass-through | GSE | REMIC      
Debt Securities, Available-for-sale [Line Items]      
Number of debt securities, unrealized loss position longer than 12 months 20    
Number of debt securities, unrealized loss position less than 12 months 23    
Internal Investment Grade | Non-GSE | REMIC      
Debt Securities, Available-for-sale [Line Items]      
Number of debt securities, unrealized loss position longer than 12 months 1