Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 5,463,186	$ 5,463,186	$ 6,152,348	$ 6,546,471	$ 6,546,471
Securities available-for-sale, Gross Unrealized Holding Gains	120		1,237	531
Securities available-for-sale, Gross Unrealized Holding Losses	(810,769)		(679,226)	(801,559)
Securities available-for-sale, Estimated Fair Value	4,652,537		5,474,359	5,745,443
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	507,549		508,275	504,457
Securities available-for-sale, Gross Unrealized Holding Gains	0		11	2
Securities available-for-sale, Gross Unrealized Holding Losses	(22,102)		(25,737)	(27,283)
Securities available-for-sale, Estimated Fair Value	485,447		482,549	477,176
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,726,947		2,104,193	2,368,605
Securities available-for-sale, Gross Unrealized Holding Gains	120		1,217	514
Securities available-for-sale, Gross Unrealized Holding Losses	(263,313)		(206,799)	(285,307)
Securities available-for-sale, Estimated Fair Value	1,463,754		1,898,611	2,083,812
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	2,792,813		3,034,120	3,153,864
Securities available-for-sale, Gross Unrealized Holding Gains	0		8	14
Securities available-for-sale, Gross Unrealized Holding Losses	(495,578)		(417,562)	(456,997)
Securities available-for-sale, Estimated Fair Value	2,297,235		2,616,566	2,696,881
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	323,539		392,914	406,530
Securities available-for-sale, Gross Unrealized Holding Gains	0		1	1
Securities available-for-sale, Gross Unrealized Holding Losses	(18,191)		(18,046)	(19,185)
Securities available-for-sale, Estimated Fair Value	305,348		374,869	387,346
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	112,338		112,846	113,015
Securities available-for-sale, Gross Unrealized Holding Gains	0		0	0
Securities available-for-sale, Gross Unrealized Holding Losses	(11,585)		(11,082)	(12,787)
Securities available-for-sale, Estimated Fair Value	$ 100,753		$ 101,764	$ 100,228