Securities - Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) $ in Thousands	Mar. 31, 2024 USD ($) Security	Dec. 31, 2023 USD ($) Security
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	382	411
Less than 12 months, Fair Value	$ 779,687	$ 734,181
Less than 12 months, Unrealized Losses	$ (13,400)	$ (9,949)
12 months or more, Count | Security	2,799	2,706
12 months or more, Fair Value	$ 5,327,697	$ 5,840,885
12 months or more, Unrealized Losses	$ (654,670)	$ (618,782)
Total Count | Security	3,181	3,117
Total Fair Value	$ 6,107,384	$ 6,575,066
Total Unrealized Losses	$ (668,070)	$ (628,731)
U.S. Treasury [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	46	8
Less than 12 months, Fair Value	$ 296,702	$ 509,946
Less than 12 months, Unrealized Losses	$ (1,282)	$ (267)
12 months or more, Count | Security	30	63
12 months or more, Fair Value	$ 417,698	$ 745,573
12 months or more, Unrealized Losses	$ (5,934)	$ (9,934)
Total Count | Security	76	71
Total Fair Value	$ 714,400	$ 1,255,519
Total Unrealized Losses	$ (7,216)	$ (10,201)
U.S. Agencies [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	10
Less than 12 months, Fair Value	$ 61,842
Less than 12 months, Unrealized Losses	$ (262)
12 months or more, Count | Security	7	16
12 months or more, Fair Value	$ 87,124	$ 116,324
12 months or more, Unrealized Losses	$ (2,486)	$ (2,937)
Total Count | Security	17	16
Total Fair Value	$ 148,966	$ 116,324
Total Unrealized Losses	$ (2,748)	$ (2,937)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	35	14
Less than 12 months, Fair Value	$ 189,438	$ 19,154
Less than 12 months, Unrealized Losses	$ (1,471)	$ (476)
12 months or more, Count | Security	848	852
12 months or more, Fair Value	$ 3,387,309	$ 3,526,296
12 months or more, Unrealized Losses	$ (538,684)	$ (508,264)
Total Count | Security	883	866
Total Fair Value	$ 3,576,747	$ 3,545,450
Total Unrealized Losses	$ (540,155)	$ (508,740)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	285	388
Less than 12 months, Fair Value	$ 193,120	$ 200,835
Less than 12 months, Unrealized Losses	$ (10,338)	$ (9,202)
12 months or more, Count | Security	1,639	1,476
12 months or more, Fair Value	$ 951,910	$ 890,545
12 months or more, Unrealized Losses	$ (79,385)	$ (65,785)
Total Count | Security	1,924	1,864
Total Fair Value	$ 1,145,030	$ 1,091,380
Total Unrealized Losses	$ (89,723)	$ (74,987)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
12 months or more, Count | Security	253	267
12 months or more, Fair Value	$ 327,473	$ 351,275
12 months or more, Unrealized Losses	$ (27,841)	$ (30,794)
Total Count | Security	253	267
Total Fair Value	$ 327,473	$ 351,275
Total Unrealized Losses	$ (27,841)	$ (30,794)
Collateralized loan obligations [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	6	1
Less than 12 months, Fair Value	$ 38,585	$ 4,246
Less than 12 months, Unrealized Losses	$ (47)	$ (4)
12 months or more, Count | Security	22	32
12 months or more, Fair Value	$ 156,183	$ 210,872
12 months or more, Unrealized Losses	$ (340)	$ (1,068)
Total Count | Security	28	33
Total Fair Value	$ 194,768	$ 215,118
Total Unrealized Losses	$ (387)	$ (1,072)