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Investment Securities (Tables)
 9 Months Ended
Sep. 30, 2025
Investments, Debt and Equity Securities [Abstract]  
Schedule of Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio
The following table summarizes the amortized cost and fair value of the available-for-sale portfolio and the corresponding amounts of gross unrealized gains, unrealized losses, and basis adjustments in accumulated other comprehensive income (loss) (“AOCI”).
(dollars in thousands)Amortized
Cost		Unrealized
Gains		Unrealized
Losses
Basis
Adjustments (1)
Fair
Value
September 30, 2025    
Available-for-Sale    
U.S. Treasury$268,957 $86 $(10,230)$(44,732)$214,081 
U.S. government-sponsored entities and agencies1,553,040 318 (146,525)(57,074)1,349,759 
Mortgage-backed securities - Agency9,636,140 68,310 (512,627)— 9,191,823 
States and political subdivisions450,185 2,073 (21,682)2,588 433,164 
Pooled trust preferred securities13,816 — (2,283)— 11,533 
Other securities222,615 1,277 (6,365)— 217,527 
Total available-for-sale securities$12,144,753 $72,064 $(699,712)$(99,218)$11,417,887 
December 31, 2024
Available-for-Sale
U.S. Treasury$261,421 $67 $(12,659)$(49,816)$199,013 
U.S. government-sponsored entities and agencies1,521,610 7 (181,360)(82,351)1,257,906 
Mortgage-backed securities - Agency5,861,067 6,005 (662,181)— 5,204,891 
States and political subdivisions510,630 148 (25,881)647 485,544 
Pooled trust preferred securities13,807 — (2,485)— 11,322 
Other securities311,973 760 (12,950)— 299,783 
Total available-for-sale securities$8,480,508 $6,987 $(897,516)$(131,520)$7,458,459 
(1)    Basis adjustments represent the amount of fair value hedging adjustments included in the carrying amounts of fixed-rate investment securities assets designated in fair value hedging arrangements. See Note 15 to the consolidated financial statements for additional information regarding these derivative financial instruments.
Schedule of Held-to-Maturity Investment Securities with Unrecognized Losses
The following table summarizes the amortized cost and fair value of the held-to-maturity investment securities portfolio and the corresponding amounts of gross unrecognized gains and losses.
(dollars in thousands)Amortized
Cost		Unrecognized
Gains		Unrecognized
Losses		Fair
Value
September 30, 2025   
Held-to-Maturity
U.S. government-sponsored entities and agencies$838,535 $— $(132,641)$705,894 
Mortgage-backed securities - Agency925,192 — (130,879)794,313 
States and political subdivisions1,146,638 182 (128,205)1,018,615 
Allowance for securities held-to-maturity(150)— — (150)
Total held-to-maturity securities$2,910,215 $182 $(391,725)$2,518,672 
December 31, 2024
Held-to-Maturity
U.S. government-sponsored entities and agencies$832,984 $— $(168,653)$664,331 
Mortgage-backed securities - Agency970,212 — (169,546)800,666 
States and political subdivisions1,151,835 317 (145,861)1,006,291 
Allowance for securities held-to-maturity(150)— — (150)
Total held-to-maturity securities$2,954,881 $317 $(484,060)$2,471,138 
The following table summarizes the held-to-maturity investment securities with unrecognized losses aggregated by major security type and length of time in a continuous loss position:
 Less than 12 months12 months or longerTotal
(dollars in thousands)Fair
Value		Unrecognized
Losses		Fair
Value		Unrecognized
Losses		Fair
Value		Unrecognized
Losses
September 30, 2025
Held-to-Maturity
U.S. government-sponsored entities
   and agencies		$— $— $705,894 $(132,641)$705,894 $(132,641)
Mortgage-backed securities - Agency— — 794,313 (130,879)794,313 (130,879)
States and political subdivisions52,126 (789)951,019 (127,416)1,003,145 (128,205)
Total held-to-maturity$52,126 $(789)$2,451,226 $(390,936)$2,503,352 $(391,725)
December 31, 2024
Held-to-Maturity
U.S. government-sponsored entities
   and agencies		$— $— $664,331 $(168,653)$664,331 $(168,653)
Mortgage-backed securities - Agency— — 800,666 (169,546)800,666 (169,546)
States and political subdivisions37,007 (430)937,364 (145,431)974,371 (145,861)
Total held-to-maturity$37,007 $(430)$2,402,361 $(483,630)$2,439,368 $(484,060)
Schedule of Proceeds from Sales or Calls and Realized Gain and Losses of Available-for-Sale Investment Securities and Other Securities
Proceeds from sales or calls of available-for-sale investment securities and the resulting realized gains and realized losses were as follows:
Three Months Ended
September 30,		Nine Months Ended
September 30,
(dollars in thousands)2025202420252024
Proceeds$57,784 $22,545 $2,221,229 $370,870 
Realized gains7 90 97 98 
Realized losses— (166)(207)(188)
Schedule of Expected Maturities of Investment Securities Portfolio
The table below shows the amortized cost and fair value of the investment securities portfolio by contractual maturity. Expected maturities may differ from contractual maturities if borrowers have the right to call or prepay obligations with or without call or prepayment penalties. Weighted average yield is based on amortized cost.
 September 30, 2025
(dollars in thousands)Amortized
Cost		Fair
Value		Weighted
Average
Yield
Maturity
Available-for-Sale   
Within one year$225,299 $224,999 3.84 %
One to five years4,387,628 4,323,449 4.48 
Five to ten years6,214,098 5,786,459 3.77 
Beyond ten years1,317,728 1,082,980 2.96 
Total$12,144,753 $11,417,887 3.94 %
Held-to-Maturity
Within one year$18,332 $18,278 3.21 %
One to five years53,952 49,913 2.03 
Five to ten years1,454,602 1,280,939 2.54 
Beyond ten years1,383,329 1,169,542 2.80 
Total$2,910,215 $2,518,672 2.66 %
Schedule of Available-for-Sale and Held-to-Maturity Investment Securities with Unrealized Losses by Aggregated Major Security Type and Length of Time in Continuous Unrealized Loss Position
The following table summarizes the available-for-sale investment securities with unrealized losses for which an allowance for credit losses has not been recorded by aggregated major security type and length of time in a continuous unrealized loss position:
 Less than 12 months12 months or longerTotal
(dollars in thousands)Fair
Value		Unrealized
Losses		Fair
Value		Unrealized
Losses		Fair
Value		Unrealized Losses
September 30, 2025
Available-for-Sale
U.S. Treasury$998 $— $184,848 $(10,230)$185,846 $(10,230)
U.S. government-sponsored entities
   and agencies		84,581 (413)1,165,864 (146,112)1,250,445 (146,525)
Mortgage-backed securities - Agency296,110 (1,726)3,247,585 (510,901)3,543,695 (512,627)
States and political subdivisions9,884 (65)240,834 (21,617)250,718 (21,682)
Pooled trust preferred securities— — 11,533 (2,283)11,533 (2,283)
Other securities17,136 (126)144,051 (6,239)161,187 (6,365)
Total available-for-sale$408,709 $(2,330)$4,994,715 $(697,382)$5,403,424 $(699,712)
December 31, 2024
Available-for-Sale
U.S. Treasury$3,977 $(26)$177,691 $(12,633)$181,668 $(12,659)
U.S. government-sponsored entities
   and agencies		98,280 (1,713)1,144,618 (179,647)1,242,898 (181,360)
Mortgage-backed securities - Agency857,440 (9,172)3,406,350 (653,009)4,263,790 (662,181)
States and political subdivisions133,906 (1,462)279,121 (24,419)413,027 (25,881)
Pooled trust preferred securities— — 11,322 (2,485)11,322 (2,485)
Other securities33,292 (295)199,631 (12,655)232,923 (12,950)
Total available-for-sale$1,126,895 $(12,668)$5,218,733 $(884,848)$6,345,628 $(897,516)