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Securities - Additional Information (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2013
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]            
Available-for-sale securities transferred to held-to-maturity securities         $ 536,000,000  
Unrealized net loss on securities transferred from available-for-sale to held-to-maturity, gross         $ 13,300,000  
Realized gain or losses on the sale of available-for-sale securities $ 0 $ 244,000 $ 0 $ 244,000    
Corporate debt securities, callable, amortized cost 29,400,000   29,400,000      
Corporate debt securities, callable, estimated fair value 31,000,000   31,000,000      
Securities pledged for other debt obligations, at fair value 993,000,000   993,000,000     $ 435,900,000
Equity investments 101,314,000   101,314,000     107,079,000
Reverse Repurchase Agreement            
Debt Securities, Available-for-sale [Line Items]            
Securities pledged for repurchase agreements, at fair value $ 150,700,000   $ 150,700,000     $ 152,700,000