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Securities Available for Sale - Summary of Securities Available for Sale (Details)
3 Months Ended 6 Months Ended
Mar. 31, 2021
USD ($)
security
Mar. 31, 2020
USD ($)
Jun. 30, 2020
Dec. 31, 2020
USD ($)
security
Debt Securities, Available-for-sale [Line Items]        
Amortized Cost $ 2,228,369,000     $ 2,239,433,000
Gross Unrealized Gains 28,368,000     48,041,000
Gross Unrealized Losses (22,993,000)     (1,863,000)
Allowance For Investment Credit Losses 0     0
Fair Value 2,233,744,000     2,285,611,000
Net gains on sales of securities available for sale 0 $ 0    
Proceeds from sale of securities available for sale 0 $ 0    
Unrealized gains on securities available for sale net of taxes $ 5,223,000     $ 32,753,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | security 98     33
Available-for-sale Securities        
Debt Securities, Available-for-sale [Line Items]        
Unrealized gains on securities available for sale net of taxes $ 4,400,000     $ 33,200,000
Collateralized mortgage obligations        
Debt Securities, Available-for-sale [Line Items]        
Amortized Cost 901,735,000     990,679,000
Gross Unrealized Gains 9,424,000     11,482,000
Gross Unrealized Losses (6,500,000)     (844,000)
Allowance For Investment Credit Losses 0     0
Fair Value 904,659,000     1,001,317,000
No single maturity date, fair value 904,659,000      
Mortgage-backed securities        
Debt Securities, Available-for-sale [Line Items]        
Allowance For Investment Credit Losses 0      
Mortgage-backed securities: Residential        
Debt Securities, Available-for-sale [Line Items]        
Amortized Cost 646,409,000     672,667,000
Gross Unrealized Gains 3,190,000     8,460,000
Gross Unrealized Losses (11,771,000)     (114,000)
Allowance For Investment Credit Losses 0     0
Fair Value $ 637,828,000     $ 681,013,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | security 44     8
No single maturity date, fair value $ 637,828,000      
Mortgage-backed securities: Commercial        
Debt Securities, Available-for-sale [Line Items]        
Amortized Cost 527,528,000     $ 482,874,000
Gross Unrealized Gains 13,378,000     25,026,000
Gross Unrealized Losses (3,118,000)     (21,000)
Allowance For Investment Credit Losses 0     0
Fair Value $ 537,788,000     $ 507,879,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | security 13     1
No single maturity date, fair value $ 537,788,000      
Corporate securities        
Debt Securities, Available-for-sale [Line Items]        
Amortized Cost 14,435,000     $ 7,000,000
Gross Unrealized Gains 0     15,000
Gross Unrealized Losses (1,230,000)     (881,000)
Allowance For Investment Credit Losses 0     0
Fair Value $ 13,205,000     $ 6,134,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | security 4     1
Municipal securities        
Debt Securities, Available-for-sale [Line Items]        
Amortized Cost $ 103,481,000     $ 86,213,000
Gross Unrealized Gains 2,272,000     3,058,000
Gross Unrealized Losses (338,000)     (3,000)
Allowance For Investment Credit Losses 0     0
Fair Value $ 105,415,000     $ 89,268,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | security 10     1
Non-US Government and Agency Securities | Credit concentration risk | Stockholders' equity        
Debt Securities, Available-for-sale [Line Items]        
Maximum exposure to any single issuer 10.00%   10.00%  
Asset-backed Securities        
Debt Securities, Available-for-sale [Line Items]        
Amortized Cost $ 34,781,000      
Gross Unrealized Gains 104,000      
Gross Unrealized Losses (36,000)      
Allowance For Investment Credit Losses 0      
Fair Value $ 34,849,000      
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions | security 1      
No single maturity date, fair value $ 34,849,000