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Investment Securities (Tables)
 9 Months Ended
Sep. 30, 2025
Investment Securities  
Schedule of amortized cost and fair value of investment securities held to maturity

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

Gross

    

Gross

​

​

 

​

​

Amortized

​

Unrealized

​

Unrealized

​

Fair

 

(Dollars in thousands)

    

Cost

    

Gains

    

Losses

    

Value

 

September 30, 2025:

​

​

​

​

​

​

​

​

​

​

​

​

​

U.S. Government agencies

​

$

132,911

​

$

—

​

$

(16,729)

​

$

116,182

​

Residential mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

1,187,818

​

​

—

​

​

(186,684)

​

​

1,001,134

​

Residential collateralized mortgage-obligations issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

386,777

​

​

—

​

​

(58,177)

​

​

328,600

​

Commercial mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

342,054

​

​

—

​

​

(60,516)

​

​

281,538

​

Small Business Administration loan-backed securities

​

​

47,167

​

​

—

​

​

(9,134)

​

​

38,033

​

​

​

$

2,096,727

​

$

—

​

$

(331,240)

​

$

1,765,487

​

December 31, 2024:

​

​

​

​

​

​

​

​

​

​

​

​

​

U.S. Government agencies

​

$

147,272

​

$

—

​

$

(23,498)

​

$

123,774

​

Residential mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

1,297,543

​

​

—

​

​

(241,204)

​

​

1,056,339

​

Residential collateralized mortgage-obligations issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

411,721

​

​

—

​

​

(72,057)

​

​

339,664

​

Commercial mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

348,338

​

​

—

​

​

(72,391)

​

​

275,947

​

Small Business Administration loan-backed securities

​

​

49,796

​

​

—

​

​

(10,993)

​

​

38,803

​

​

​

$

2,254,670

​

$

—

​

$

(420,143)

​

$

1,834,527

​
Schedule of amortized cost and fair value of investment securities available for sale

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

Gross

​

Gross

​

​

​

​

​

​

Amortized

​

Unrealized

​

Unrealized

​

Fair

 

(Dollars in thousands)

    

Cost

    

Gains

    

Losses

    

Value

 

September 30, 2025:

​

​

​

​

​

​

​

​

​

​

​

​

​

Residential mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

$

1,689,961

​

$

7,641

​

$

(147,237)

​

$

1,550,365

​

Residential collateralized mortgage-obligations issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

2,268,056

​

​

18,908

​

​

(49,790)

​

​

2,237,174

​

Commercial mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

749,857

​

​

2,809

​

​

(77,054)

​

​

675,612

​

State and municipal obligations

​

 

1,112,505

​

 

754

​

 

(152,147)

​

 

961,112

​

Small Business Administration loan-backed securities

​

 

619,437

​

 

528

​

 

(28,405)

​

 

591,560

​

Corporate securities

​

​

28,499

​

​

1

​

​

(1,523)

​

​

26,977

​

​

​

$

6,468,315

​

$

30,641

​

$

(456,156)

​

$

6,042,800

​

December 31, 2024:

​

​

​

​

​

​

​

​

​

​

​

​

​

U.S. Treasuries

​

$

10,654

​

$

2

​

$

—

​

$

10,656

​

U.S. Government agencies

​

​

169,207

​

​

—

​

​

(18,789)

​

​

150,418

​

Residential mortgage-backed securities issued by U.S. government

​

 

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

1,659,851

​

 

97

​

 

(282,423)

​

 

1,377,525

​

Residential collateralized mortgage-obligations issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

557,288

​

​

19

​

​

(98,212)

​

​

459,095

​

Commercial mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

1,234,573

​

​

562

​

​

(194,580)

​

​

1,040,555

​

State and municipal obligations

​

​

1,117,330

​

 

2

​

 

(171,609)

​

 

945,723

​

Small Business Administration loan-backed securities

​

 

351,814

​

 

19

​

 

(41,721)

​

 

310,112

​

Corporate securities

​

 

28,499

​

​

—

​

​

(1,990)

​

​

26,509

​

​

​

$

5,129,216

​

$

701

​

$

(809,324)

​

$

4,320,593

​
Schedule of amortized cost and carrying value of other investment securities

​

​

​

​

​

​

​

​

Carrying

 

(Dollars in thousands)

    

Value

 

September 30, 2025:

​

​

​

​

Federal Home Loan Bank stock

​

$

18,086

​

Federal Reserve Bank stock

​

​

234,374

​

Investment in unconsolidated subsidiaries

​

 

5,287

​

Other investment securities

​

 

108,471

​

​

​

$

366,218

​

December 31, 2024:

​

​

​

​

Federal Home Loan Bank stock

​

$

18,087

​

Federal Reserve Bank stock

​

​

150,261

​

Investment in unconsolidated subsidiaries

​

 

3,563

​

Other investment securities

​

 

51,702

​

​

​

$

223,613

​
Schedule of amortized cost and fair value of debt and equity securities by contractual maturity

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

Securities

​

Securities

 

​

​

Held to Maturity

​

Available for Sale

 

​

​

Amortized

​

Fair

​

Amortized

​

Fair

 

(Dollars in thousands)

    

Cost

    

Value

    

Cost

    

Value

 

Due in one year or less

    

$

—

​

$

—

    

$

34,325

    

$

34,214

​

Due after one year through five years

​

 

94,797

​

 

87,406

​

 

323,617

​

 

322,072

​

Due after five years through ten years

​

 

381,000

​

 

337,485

​

 

800,111

​

 

744,473

​

Due after ten years

​

 

1,620,930

​

 

1,340,596

​

 

5,310,262

​

 

4,942,041

​

​

​

$

2,096,727

​

$

1,765,487

​

$

6,468,315

​

$

6,042,800

​
Schedule of information with respect to sales of available-for-sale securities

​

​

​

​

​

​

​

​

​

​

​

​

​

​

Nine Months Ended September 30,

​

​

​

2025

 

(Dollars in thousands)

​

Sales of Securities Acquired from Independent

​

Investment Securities Sales

​

Total

​

Sale proceeds

​

$

1,279,717

    

$

1,594,393

​

$

2,874,110

​

Gross realized gains

​

​

—

​

​

8,892

​

​

8,892

​

Gross realized losses

​

 

—

​

 

(237,703)

​

 

(237,703)

​

Net realized losses

​

$

—

​

$

(228,811)

​

$

(228,811)

​
Schedule of securities with gross unrealized losses, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

Less Than

​

Twelve Months

 

​

​

Twelve Months

​

or More

 

​

​

Gross Unrealized

​

Fair

​

Gross Unrealized

​

Fair

 

(Dollars in thousands)

    

Losses

    

Value

    

Losses

    

Value

 

September 30, 2025:

​

​

​

​

​

​

​

​

​

​

​

​

​

Securities Held to Maturity

​

​

​

​

​

​

​

​

​

​

​

​

​

U.S. Government agencies

​

$

—

​

$

—

​

$

16,729

​

$

116,182

​

Residential mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

—

​

​

—

​

​

186,684

​

​

1,001,134

​

Residential collateralized mortgage-obligations issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

—

​

 

—

​

 

58,177

​

 

328,600

​

Commercial mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

—

​

​

—

​

​

60,516

​

​

281,538

​

Small Business Administration loan-backed securities

​

​

—

​

​

—

​

​

9,134

​

​

38,033

​

​

​

$

—

​

$

—

​

$

331,240

​

$

1,765,487

​

Securities Available for Sale

​

​

​

​

​

​

​

​

​

​

​

​

​

Residential mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

$

484

​

$

64,033

​

$

146,753

​

$

839,764

​

Residential collateralized mortgage-obligations issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

121

​

 

102,208

​

 

49,669

​

 

302,001

​

Commercial mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

 

268

​

​

56,511

​

​

76,786

​

​

415,144

​

State and municipal obligations

​

 

372

​

​

9,326

​

​

151,775

​

​

900,630

​

Small Business Administration loan-backed securities

​

 

1,520

​

​

318,038

​

​

26,885

​

​

205,350

​

Corporate securities

​

​

—

​

​

—

​

​

1,523

​

​

26,477

​

​

​

$

2,765

​

$

550,116

​

$

453,391

​

$

2,689,366

​

December 31, 2024:

​

​

​

​

​

​

​

​

​

​

​

​

​

Securities Held to Maturity

​

​

​

​

​

​

​

​

​

​

​

​

​

U.S. Government agencies

​

$

—

​

$

—

​

$

23,498

​

$

123,774

​

Residential mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

—

​

​

—

​

​

241,204

​

​

1,056,339

​

Residential collateralized mortgage-obligations issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

—

​

 

—

​

 

72,057

​

 

339,664

​

Commercial mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

—

​

​

—

​

​

72,391

​

​

275,947

​

Small Business Administration loan-backed securities

​

​

—

​

​

—

​

​

10,993

​

​

38,803

​

​

​

$

—

​

$

—

​

$

420,143

​

$

1,834,527

​

Securities Available for Sale

​

​

​

​

​

​

​

​

​

​

​

​

​

U.S. Treasuries

​

$

—

​

$

—

​

$

—

​

$

—

​

U.S. Government agencies

​

​

—

​

​

—

​

​

18,789

​

​

150,418

​

Residential mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

294

​

​

14,341

​

​

282,129

​

​

1,350,268

​

Residential collateralized mortgage-obligations issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

—

​

 

—

​

 

98,212

​

 

454,908

​

Commercial mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

792

​

​

53,342

​

​

193,788

​

​

918,338

​

State and municipal obligations

​

​

1,484

​

​

19,400

​

​

170,125

​

​

923,431

​

Small Business Administration loan-backed securities

​

​

24

​

​

6,747

​

​

41,697

​

​

289,786

​

Corporate securities

​

 

—

​

​

—

​

​

1,990

​

​

26,509

​

​

​

$

2,594

​

$

93,830

​

$

806,730

​

$

4,113,658

​
Schedule of trading securities

​

​

​

​

​

​

​

​

​

​

    

September 30,

​

December 31,

​

(Dollars in thousands)

    

2025

 

2024

​

U.S. Government agencies

​

$

16,480

​

$

15,002

​

Residential mortgage pass-through securities issued or guaranteed by U.S.

​

​

​

​

​

​

​

government agencies or sponsored enterprises

​

​

5,753

​

​

14,803

​

Other residential mortgage issued or guaranteed by U.S. government

​

 

​

​

 

​

​

agencies or sponsored enterprises

​

 

5,017

​

 

—

​

Commercial mortgage-backed securities issued by U.S. government

​

​

​

​

​

​

​

agencies or sponsored enterprises

​

​

15,222

​

​

14,419

​

State and municipal obligations

​

​

37,195

​

​

35,896

​

Small Business Administration loan-backed securities

​

​

27,285

​

​

22,571

​

Other debt securities

​

​

567

​

​

241

​

​

​

$

107,519

​

$

102,932

​