Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 7,940,236	$ 10,234,258	$ 9,198,374	$ 8,687,276
Securities held to maturity, net of allowance for credit losses of $500 at September 30, 2025 and $310 at December 31, 2024	3,454,732	3,533,775
Deposits	42,517,152	42,722,748
Federal funds purchased	31,775	21,975
Repurchase agreements	4,563,749	4,342,941
Junior subordinated deferrable interest debentures	123,227	123,184
Subordinated notes	99,765	99,648
Loans, net	21,165,353	20,484,662
Carrying Amount [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7,940,236	10,234,258
Securities held to maturity, net of allowance for credit losses of $500 at September 30, 2025 and $310 at December 31, 2024	3,454,732	3,533,775
Accrued interest receivable	206,622	236,591
Deposits	42,517,152	42,722,748
Federal funds purchased	31,775	21,975
Repurchase agreements	4,563,749	4,342,941
Junior subordinated deferrable interest debentures	123,227	123,184
Subordinated notes	99,765	99,648
Accrued interest payable	50,213	58,870
Carrying Amount [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	21,165,353	20,484,662
Estimated Fair Value [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7,940,236	10,234,258
Securities held to maturity, net of allowance for credit losses of $500 at September 30, 2025 and $310 at December 31, 2024	3,288,919	3,360,546
Accrued interest receivable	206,622	236,591
Deposits	42,507,046	42,712,907
Federal funds purchased	31,775	21,975
Repurchase agreements	4,563,749	4,342,941
Junior subordinated deferrable interest debentures	123,712	123,712
Subordinated notes	99,500	98,453
Accrued interest payable	50,213	58,870
Estimated Fair Value [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 20,917,289	$ 20,066,512

CULLEN/FROST BANKERS, INC. 10-Q