Securities - Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) $ in Thousands	Jun. 30, 2021 USD ($) Security	Dec. 31, 2020 USD ($) Security
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	496	44
Less than 12 months, Fair Value	$ 3,351,446	$ 235,222
Less than 12 months, Unrealized Losses	$ (41,678)	$ (1,452)
12 months or more, Count | Security	3	6
12 months or more, Fair Value	$ 1,850	$ 2,833
12 months or more, Unrealized Losses	$ (178)	$ (259)
Total Count | Security	499	50
Total Fair Value	$ 3,353,296	$ 238,055
Total Unrealized Losses	$ (41,856)	$ (1,711)
U.S. Treasury [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	1
Less than 12 months, Fair Value	$ 4,860
Less than 12 months, Unrealized Losses	$ (34)
Total Count | Security	1
Total Fair Value	$ 4,860
Total Unrealized Losses	$ (34)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	180	16
Less than 12 months, Fair Value	$ 3,011,311	$ 174,234
Less than 12 months, Unrealized Losses	$ (35,787)	$ (1,157)
Total Count | Security	180	16
Total Fair Value	$ 3,011,311	$ 174,234
Total Unrealized Losses	$ (35,787)	$ (1,157)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	310	24
Less than 12 months, Fair Value	$ 324,484	$ 55,653
Less than 12 months, Unrealized Losses	$ (5,780)	$ (279)
12 months or more, Count | Security	3	6
12 months or more, Fair Value	$ 1,850	$ 2,833
12 months or more, Unrealized Losses	$ (178)	$ (259)
Total Count | Security	313	30
Total Fair Value	$ 326,334	$ 58,486
Total Unrealized Losses	$ (5,958)	$ (538)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	5	4
Less than 12 months, Fair Value	$ 10,791	$ 5,335
Less than 12 months, Unrealized Losses	$ (77)	$ (16)
Total Count | Security	5	4
Total Fair Value	$ 10,791	$ 5,335
Total Unrealized Losses	$ (77)	$ (16)