Investment Securities - Available-for-Sale Investment Securities with Unrealized Losses by Aggregated Major Security Type and Length of Time in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	$ 242,995	$ 1,295,822
Available-for-Sale, Less than 12 Months, Unrealized Losses	(4,163)	(11,960)
Available-for-Sale, 12 months or longer, Fair Value	45,885	314,350
Available-for-Sale, 12 months or longer, Unrealized Losses	(8,535)	(9,700)
Available-for-Sale, Fair Value	288,880	1,610,172
Available-for-Sale, Unrealized Losses	(12,698)	(21,660)
U.S. Treasury
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value		999
Available-for-Sale, Less than 12 Months, Unrealized Losses		(2)
Available-for-Sale, 12 months or longer, Fair Value		0
Available-for-Sale, 12 months or longer, Unrealized Losses		0
Available-for-Sale, Fair Value		999
Available-for-Sale, Unrealized Losses	0	(2)
U.S. government-sponsored entities and agencies
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value		357,647
Available-for-Sale, Less than 12 Months, Unrealized Losses		(4,638)
Available-for-Sale, 12 months or longer, Fair Value		0
Available-for-Sale, 12 months or longer, Unrealized Losses		0
Available-for-Sale, Fair Value		357,647
Available-for-Sale, Unrealized Losses	0	(4,638)
Mortgage-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	66,182	786,245
Available-for-Sale, Less than 12 Months, Unrealized Losses	(295)	(6,122)
Available-for-Sale, 12 months or longer, Fair Value	13,308	212,056
Available-for-Sale, 12 months or longer, Unrealized Losses	(199)	(2,930)
Available-for-Sale, Fair Value	79,490	998,301
Available-for-Sale, Unrealized Losses	(494)	(9,052)
States and political subdivisions
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	75,669	120,166
Available-for-Sale, Less than 12 Months, Unrealized Losses	(961)	(1,016)
Available-for-Sale, 12 months or longer, Fair Value	465	7,006
Available-for-Sale, 12 months or longer, Unrealized Losses	0	(31)
Available-for-Sale, Fair Value	76,134	127,172
Available-for-Sale, Unrealized Losses	(961)	(1,047)
Pooled trust preferred securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	0	0
Available-for-Sale, Less than 12 Months, Unrealized Losses	0	0
Available-for-Sale, 12 months or longer, Fair Value	7,422	8,222
Available-for-Sale, 12 months or longer, Unrealized Losses	(6,376)	(5,589)
Available-for-Sale, Fair Value	7,422	8,222
Available-for-Sale, Unrealized Losses	(6,376)	(5,589)
Other securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	101,144	30,765
Available-for-Sale, Less than 12 Months, Unrealized Losses	(2,907)	(182)
Available-for-Sale, 12 months or longer, Fair Value	24,690	87,066
Available-for-Sale, 12 months or longer, Unrealized Losses	(1,960)	(1,150)
Available-for-Sale, Fair Value	125,834	117,831
Available-for-Sale, Unrealized Losses	$ (4,867)	$ (1,332)