Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 11,248,458	$ 11,248,458	$ 11,193,152
Investment securities available for sale, gross unrealized gains	234		515
Investment securities available for sale, gross unrealized losses	441,033		444,706
Investment securities available for sale, estimated fair value	10,807,659		10,748,961
Amortized cost for held to maturity	15,918,106		13,529,969
Gross unrealized gains for held to maturity	11,258		10,288
Gross unrealized losses for held to maturity	1,215,868		1,164,837
Estimated fair value for held to maturity	14,713,496	14,713,496	12,375,420
Equity and other securities, Amortized Cost	1,191,927	$ 1,191,927	933,766
Equity securities, Gross Unrealized Gains	2,137		2,120
Equity securities, Gross Unrealized Losses	3,374		3,945
Equity and other securities, Estimated Fair Value	1,190,690		931,941
Other securities, Amortized cost	972,522		780,483
Other securities, Estimated fair value	972,522		780,483
Total debt securities Amortized cost	27,166,564		24,723,121
Total debt securities Gross unrealized gains	11,492		10,803
Total debt securities Gross unrealized losses	1,656,901		1,609,543
Total debt securities Estimated fair value	25,521,155		23,124,381
Readily marketable securities Amortized cost	219,405		153,283
Readily marketable securities Gross unrealized gains	2,137		2,120
Readily marketable securities Gross unrealized losses	3,374		3,945
Readily marketable securities Estimated fair value	218,168		151,458
U.S. Treasury and Federal Agencies [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	7,930,292		7,913,932
Investment securities available for sale, gross unrealized gains	18		200
Investment securities available for sale, gross unrealized losses	237,858		243,172
Investment securities available for sale, estimated fair value	7,692,452		7,670,960
Amortized cost for held to maturity	1,049,342		1,054,035
Gross unrealized losses for held to maturity	48,391		45,747
Estimated fair value for held to maturity	1,000,951		1,008,288
Obligations of States and Political Subdivisions [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Amortized cost for held to maturity	2,549,551		2,577,078
Gross unrealized gains for held to maturity	2,009		4
Gross unrealized losses for held to maturity	103,344		116,512
Estimated fair value for held to maturity	2,448,216		2,460,570
Commercial [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	634,961		594,779
Investment securities available for sale, gross unrealized losses	19,998		20,480
Investment securities available for sale, estimated fair value	614,963		574,299
Amortized cost for held to maturity	2,029,755		912,431
Gross unrealized losses for held to maturity	148,315		103,528
Estimated fair value for held to maturity	1,881,440		808,903
Residential [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	2,506,882		2,501,334
Investment securities available for sale, gross unrealized gains	105		65
Investment securities available for sale, gross unrealized losses	166,959		171,281
Investment securities available for sale, estimated fair value	2,340,028		2,330,118
Amortized cost for held to maturity	10,241,181		8,934,918
Gross unrealized gains for held to maturity			1,451
Gross unrealized losses for held to maturity	909,050		891,063
Estimated fair value for held to maturity	9,332,131		8,045,306
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	176,323		183,107
Investment securities available for sale, gross unrealized gains	111		250
Investment securities available for sale, gross unrealized losses	16,218		9,773
Investment securities available for sale, estimated fair value	160,216		173,584
Amortized cost for held to maturity	1,641		1,765
Estimated fair value for held to maturity	1,641		1,765
Privately Issued [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Amortized cost for held to maturity	46,636		49,742
Gross unrealized gains for held to maturity	9,249		8,833
Gross unrealized losses for held to maturity	6,768		7,987
Estimated fair value for held to maturity	$ 49,117		$ 50,588

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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