Securities - Gross Unrealized Losses and Fair Value of Investment Securities Available for Sale (Detail) $ in Thousands	Mar. 31, 2022 USD ($) Security	Dec. 31, 2021 USD ($) Security
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	2,248	858
Less than 12 months, Fair Value	$ 6,146,151	$ 5,997,925
Less than 12 months, Unrealized Losses	$ (354,755)	$ (96,520)
12 months or more, Count | Security	74	44
12 months or more, Fair Value	$ 277,226	$ 278,234
12 months or more, Unrealized Losses	$ (28,388)	$ (10,748)
Total Count | Security	2,322	902
Total Fair Value	$ 6,423,377	$ 6,276,159
Total Unrealized Losses	$ (383,143)	$ (107,268)
U.S. Treasury [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	35	4
Less than 12 months, Fair Value	$ 364,450	$ 58,867
Less than 12 months, Unrealized Losses	$ (6,789)	$ (751)
Total Count | Security	35	4
Total Fair Value	$ 364,450	$ 58,867
Total Unrealized Losses	$ (6,789)	$ (751)
U.S. Agencies [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	5	1
Less than 12 months, Fair Value	$ 63,274	$ 11,149
Less than 12 months, Unrealized Losses	$ (1,265)	$ (1)
Total Count | Security	5	1
Total Fair Value	$ 63,274	$ 11,149
Total Unrealized Losses	$ (1,265)	$ (1)
Mortgage-backed [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	659	344
Less than 12 months, Fair Value	$ 4,316,175	$ 5,404,968
Less than 12 months, Unrealized Losses	$ (269,158)	$ (87,301)
12 months or more, Count | Security	12	13
12 months or more, Fair Value	$ 207,738	$ 233,295
12 months or more, Unrealized Losses	$ (17,657)	$ (8,571)
Total Count | Security	671	357
Total Fair Value	$ 4,523,913	$ 5,638,263
Total Unrealized Losses	$ (286,815)	$ (95,872)
State and political subdivisions [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	1,302	357
Less than 12 months, Fair Value	$ 1,043,982	$ 329,042
Less than 12 months, Unrealized Losses	$ (62,299)	$ (6,969)
12 months or more, Count | Security	60	31
12 months or more, Fair Value	$ 65,444	$ 44,939
12 months or more, Unrealized Losses	$ (10,631)	$ (2,177)
Total Count | Security	1,362	388
Total Fair Value	$ 1,109,426	$ 373,981
Total Unrealized Losses	$ (72,930)	$ (9,146)
Corporates [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	241	152
Less than 12 months, Fair Value	$ 327,050	$ 193,899
Less than 12 months, Unrealized Losses	$ (15,230)	$ (1,498)
12 months or more, Count | Security	2
12 months or more, Fair Value	$ 4,044
12 months or more, Unrealized Losses	$ (100)
Total Count | Security	243	152
Total Fair Value	$ 331,094	$ 193,899
Total Unrealized Losses	$ (15,330)	$ (1,498)
Collateralized loan obligations [Member]
Gain (Loss) on Investments [Line Items]
Less than 12 months, Count | Security	6
Less than 12 months, Fair Value	$ 31,220
Less than 12 months, Unrealized Losses	$ (14)
Total Count | Security	6
Total Fair Value	$ 31,220
Total Unrealized Losses	$ (14)