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Investments Narrative (Details) - USD ($)
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Jan. 01, 2020
Dec. 31, 2019
Percentage of short-term investments in interest-bearing deposits, securities purchased under agreements to resell, and federal funds sold assets rated below triple-B 0.00%      
Allowance for credit losses for securities purchased under agreements to resell $ 0     $ 0.0
Net unrealized gains (losses) on trading securities 46,120,000 $ (274,000)    
Increase in the allowance for credit losses as a cumulative-effect adjustment to retained earnings upon adoption of ASU 2016-13 $ (7,530,000) 0    
Net credit losses recognized under previous accounting metodology   $ 103,000    
Available-for-sale securities rated single-A or above, excluding private-label MBS based on amortized cost 99.80%      
Held-to-maturity securities rated single-A or above, excluding private-label MBS based on amortized cost 92.40%      
Debt Securities, Available-for-sale, Allowance for Credit Loss $ 0      
Allowance for credit loss on any HTM securities excluding private-label MBS 0.0      
Interest Bearing Deposits And Federal Funds Sold [Member]        
Allowance for credit loss for interest-bearing deposits and federal funds 0     0.0
Interest-bearing Deposits [Member]        
Accrued interest receivable 294,000     682,000
Federal Funds Sold [Member]        
Accrued interest receivable 8,000     37,000
Securities Borrowed Or Purchased Under Agreements To Resell [Member]        
Accrued interest receivable $ 1,000     $ 348,000
Private-label [Member]        
Private-label MBS based on amortized cost rated single-A or above 0.00%      
Held-to-maturity Securities [Member] | Private-label [Member]        
Net decrease in allowance for credit losses $ 1,500,000      
Cumulative Effect Period Of Adoption Adjustment [Member] | Accounting Standards Update 2016-13 [Member]        
Increase in the allowance for credit losses as a cumulative-effect adjustment to retained earnings upon adoption of ASU 2016-13     $ 7,500,000  
Cumulative Effect Period Of Adoption Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | Held-to-maturity Securities [Member] | Private-label [Member]        
Increase in the allowance for credit losses as a cumulative-effect adjustment to retained earnings upon adoption of ASU 2016-13     $ 5,300,000