Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Schedule Of Marketable Securities [Line Items]
Carrying value of pledged securities held to maturity	$ 2,174,084		$ 2,174,084		$ 1,749,179
AOCI, Debt Securities, Available-for-sale, Adjustment, Pre Tax	2,000		2,000		2,500
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	$ 1,600		$ 1,600		2,000
Percent Municipal Bond Portfolio Issued By Political Subdivision Or Agencies Within State Of Texas Guaranteed By Texas Permanent School Fund Or Have Been Pre-Refunded	75.10%		75.10%
Carrying value of pledged securities, available-for-sale	$ 17,399,107		$ 17,399,107		13,924,628
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0		0
Proceeds from Sale of Held-to-maturity Securities	0	$ 0	0	$ 0
Proceeds from Sale of Debt Securities, Available-for-sale	0	$ 0	0	$ 0
Held-to-maturity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Accrued interest receivable	11,500		11,500		18,400
Held-to-maturity Securities [Member] | Collateralized Securities
Schedule Of Marketable Securities [Line Items]
Carrying value of pledged securities held to maturity	271,600		271,600		642,300
Available-for-sale Securities [Member]
Schedule Of Marketable Securities [Line Items]
Accrued interest receivable	99,700		99,700		120,500
Available-for-sale Securities [Member] | Collateralized Securities
Schedule Of Marketable Securities [Line Items]
Carrying value of pledged securities, available-for-sale	$ 4,900,000		$ 4,900,000		$ 5,800,000