Securities - Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) $ in Thousands	Mar. 31, 2021 USD ($) Security	Dec. 31, 2020 USD ($) Security
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	716	44
Less than 12 months, Fair Value	$ 3,476,682	$ 235,222
Less than 12 months, Unrealized Losses	$ (95,566)	$ (1,452)
12 months or more, Count | Security	4	6
12 months or more, Fair Value	$ 2,531	$ 2,833
12 months or more, Unrealized Losses	$ (202)	$ (259)
Total Count | Security	720	50
Total Fair Value	$ 3,479,213	$ 238,055
Total Unrealized Losses	$ (95,768)	$ (1,711)
U.S. Treasury [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	3
Less than 12 months, Fair Value	$ 14,161
Less than 12 months, Unrealized Losses	$ (345)
Total Count | Security	3
Total Fair Value	$ 14,161
Total Unrealized Losses	$ (345)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	168	16
Less than 12 months, Fair Value	$ 2,871,680	$ 174,234
Less than 12 months, Unrealized Losses	$ (79,559)	$ (1,157)
Total Count | Security	168	16
Total Fair Value	$ 2,871,680	$ 174,234
Total Unrealized Losses	$ (79,559)	$ (1,157)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	519	24
Less than 12 months, Fair Value	$ 565,317	$ 55,653
Less than 12 months, Unrealized Losses	$ (15,477)	$ (279)
12 months or more, Count | Security	4	6
12 months or more, Fair Value	$ 2,531	$ 2,833
12 months or more, Unrealized Losses	$ (202)	$ (259)
Total Count | Security	523	30
Total Fair Value	$ 567,848	$ 58,486
Total Unrealized Losses	$ (15,679)	$ (538)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	26	4
Less than 12 months, Fair Value	$ 25,524	$ 5,335
Less than 12 months, Unrealized Losses	$ (185)	$ (16)
Total Count | Security	26	4
Total Fair Value	$ 25,524	$ 5,335
Total Unrealized Losses	$ (185)	$ (16)