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Securities - Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail)
$ in Thousands
Sep. 30, 2021
USD ($)
Security
Dec. 31, 2020
USD ($)
Security
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 792 44
Less than 12 months, Fair Value $ 4,575,288 $ 235,222
Less than 12 months, Unrealized Losses $ (76,108) $ (1,452)
12 months or more, Count | Security 21 6
12 months or more, Fair Value $ 61,595 $ 2,833
12 months or more, Unrealized Losses $ (2,948) $ (259)
Total Count | Security 813 50
Total Fair Value $ 4,636,883 $ 238,055
Total Unrealized Losses $ (79,056) $ (1,711)
U.S. Treasury [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 4  
Less than 12 months, Fair Value $ 59,419  
Less than 12 months, Unrealized Losses $ (171)  
Total Count | Security 4  
Total Fair Value $ 59,419  
Total Unrealized Losses $ (171)  
Mortgage-backed [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 249 16
Less than 12 months, Fair Value $ 3,939,967 $ 174,234
Less than 12 months, Unrealized Losses $ (64,932) $ (1,157)
12 months or more, Count | Security 2  
12 months or more, Fair Value $ 27,627  
12 months or more, Unrealized Losses $ (1,232)  
Total Count | Security 251 16
Total Fair Value $ 3,967,594 $ 174,234
Total Unrealized Losses $ (66,164) $ (1,157)
State and political subdivisions [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 513 24
Less than 12 months, Fair Value $ 539,497 $ 55,653
Less than 12 months, Unrealized Losses $ (10,792) $ (279)
12 months or more, Count | Security 19 6
12 months or more, Fair Value $ 33,968 $ 2,833
12 months or more, Unrealized Losses $ (1,716) $ (259)
Total Count | Security 532 30
Total Fair Value $ 573,465 $ 58,486
Total Unrealized Losses $ (12,508) $ (538)
Corporates [Member]    
Gain (Loss) on Investments [Line Items]    
Less than 12 months, Count | Security 26 4
Less than 12 months, Fair Value $ 36,405 $ 5,335
Less than 12 months, Unrealized Losses $ (213) $ (16)
Total Count | Security 26 4
Total Fair Value $ 36,405 $ 5,335
Total Unrealized Losses $ (213) $ (16)