Securities (Year End Securities with Unrealized Losses Segregated by Length of Impairment) (Detail) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 864,035
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9,387
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	9,706,991
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,117,430
Available for Sale, Total Estimated Fair Value	10,571,026
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,126,817	$ 1,563,378
US Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,407,177
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	148,357
Available for Sale, Total Estimated Fair Value	2,407,177
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	148,357	249,895
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	65,522
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	192
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	4,540,847
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	750,821
Available for Sale, Total Estimated Fair Value	4,606,369
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	751,013	1,029,154
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	798,513
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9,195
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,758,967
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	218,252
Available for Sale, Total Estimated Fair Value	3,557,480
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 227,447	$ 284,329

CULLEN/FROST BANKERS, INC. 10-K