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Securities - Securities Available for Sale (Detail) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 10,956,847 $ 8,887,734
Gross Unrealized Gains 284,899 413,665
Gross Unrealized Losses (79,056) (1,711)
Fair Value 11,162,690 9,299,688
U.S. Treasury [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 69,515 29,911
Gross Unrealized Gains 515 829
Gross Unrealized Losses (171)  
Fair Value 69,859 30,740
U.S. Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 89,113 89,554
Gross Unrealized Gains 4,542 6,395
Fair Value 93,655 95,949
Mortgage-backed [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 7,309,736 5,266,394
Gross Unrealized Gains 126,649 202,944
Gross Unrealized Losses (66,164) (1,157)
Fair Value 7,370,221 5,468,181
State and political subdivisions [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,324,563 3,424,309
Gross Unrealized Gains 149,848 199,848
Gross Unrealized Losses (12,508) (538)
Fair Value 3,461,903 3,623,619
Corporates [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 163,920 77,566
Gross Unrealized Gains 3,345 3,649
Gross Unrealized Losses (213) (16)
Fair Value $ 167,052 $ 81,199