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Investment Securities - Additional Information (Detail)
 3 Months Ended 9 Months Ended
Mar. 31, 2024
USD ($)
security
Mar. 31, 2023
USD ($)
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Schedule of Available-for-sale Securities [Line Items]        
Carrying value of investment securities $ 3,550,000,000   $ 3,570,000,000  
Investment securities pledged as collateral 176,100,000   142,100,000  
Available for sale securities sold 0 $ 0    
Provision for credit loss 0 0 1,683,000  
Investment securities, provision for credit losses 2,525,000 842,000 2,525,000 $ 842,000
Allowance for credit loss, held-to-maturity 2,005,000 $ 2,005,000 2,005,000 $ 2,005,000
Debt securities available for sale unrealized loss position 364,793,000   336,279,000  
Fair value of unrealized losses $ 361,287,000   333,485,000  
Percentage of Company's investment portfolio 34.70%      
Maturity description of investment portfolio five years or less      
Number of investment securities available for sale | security 1,570      
Number of investment in debt securities available-for-sale unrealized loss position | security 1,321      
Number of investment securities held to maturity | security 508      
Unrealized losses $ 121,657,000   112,063,000  
Number of investments in debt securities held-to-maturity unrealized loss position | security 492      
U.S. government-sponsored enterprises        
Schedule of Available-for-sale Securities [Line Items]        
Investment securities, provision for credit losses $ 0   0  
Allowance for credit loss, held-to-maturity 0   0  
Debt securities available for sale unrealized loss position 18,537,000   17,093,000  
Fair value of unrealized losses $ 18,008,000   17,091,000  
Number of investment in debt securities available-for-sale unrealized loss position | security 63      
Unrealized losses $ 3,342,000   2,607,000  
Number of investments in debt securities held-to-maturity unrealized loss position | security 5      
U.S. government-sponsored mortgage-backed securities        
Schedule of Available-for-sale Securities [Line Items]        
Investment securities, provision for credit losses $ 0   0  
Allowance for credit loss, held-to-maturity 0   0  
Debt securities available for sale unrealized loss position 215,187,000   191,557,000  
Fair value of unrealized losses $ 214,231,000   189,391,000  
Number of investment in debt securities available-for-sale unrealized loss position | security 660      
Unrealized losses $ 6,093,000   4,362,000  
Number of investments in debt securities held-to-maturity unrealized loss position | security 20      
Private mortgage-backed securities        
Schedule of Available-for-sale Securities [Line Items]        
Investment securities, provision for credit losses $ 0   0  
Debt securities available for sale unrealized loss position 16,614,000   16,117,000  
Fair value of unrealized losses $ 16,614,000   15,891,000  
Number of investment in debt securities available-for-sale unrealized loss position | security 32      
Non-government-sponsored asset backed securities        
Schedule of Available-for-sale Securities [Line Items]        
Investment securities, provision for credit losses $ 0    
Debt securities available for sale unrealized loss position 6,421,000   7,551,000  
Fair value of unrealized losses $ 6,406,000   7,551,000  
Number of investment in debt securities available-for-sale unrealized loss position | security 33      
State and political subdivisions        
Schedule of Available-for-sale Securities [Line Items]        
Investment securities, provision for credit losses $ 0   0  
Allowance for credit loss, held-to-maturity 2,005,000   2,005,000  
Debt securities available for sale unrealized loss position 81,647,000   75,931,000  
Fair value of unrealized losses $ 80,726,000   75,531,000  
Number of investment in debt securities available-for-sale unrealized loss position | security 471      
Unrealized losses $ 112,222,000   105,094,000  
Number of investments in debt securities held-to-maturity unrealized loss position | security 467      
Other securities        
Schedule of Available-for-sale Securities [Line Items]        
Investment securities, provision for credit losses $ 2,525,000   2,525,000  
Debt securities available for sale unrealized loss position 26,387,000   28,030,000  
Fair value of unrealized losses $ 25,302,000   $ 28,030,000  
Number of investment in debt securities available-for-sale unrealized loss position | security 62