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Debt Securities (Tables)
 12 Months Ended
Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]  
Amortized cost, gross unrealized gains and losses and the fair value of debt securities
The following tables present the amortized cost, the gross unrealized gains and losses and the fair value of the Company’s debt securities:
 December 31, 2023
 Amortized
Cost		Gross Unrealized GainsGross Unrealized LossesFair
Value
(Dollars in thousands)
Available-for-sale
U.S. government and federal agency$485,005 11 (29,669)455,347 
U.S. government sponsored enterprises321,993 — (22,774)299,219 
State and local governments101,903 302 (3,273)98,932 
Corporate bonds27,007 2 (756)26,253 
Residential mortgage-backed securities3,166,589 7 (355,333)2,811,263 
Commercial mortgage-backed securities1,180,756 519 (86,570)1,094,705 
Total available-for-sale5,283,253 841 (498,375)4,785,719 
Held-to-maturity
U.S. government and federal agency853,273 — (65,472)787,801 
State and local governments1,650,000 2,843 (181,192)1,471,651 
Residential mortgage-backed securities999,138 — (78,396)920,742 
Total held-to-maturity3,502,411 2,843 (325,060)3,180,194 
Total debt securities$8,785,664 3,684 (823,435)7,965,913 
 December 31, 2022
 Amortized
Cost		Gross Unrealized GainsGross Unrealized LossesFair
Value
(Dollars in thousands)
Available-for-sale
U.S. government and federal agency$487,320 23 (42,616)444,727 
U.S. government sponsored enterprises320,157 — (32,793)287,364 
State and local governments137,033 709 (4,749)132,993 
Corporate bonds27,101 — (992)26,109 
Residential mortgage-backed securities3,706,427 6 (439,092)3,267,341 
Commercial mortgage-backed securities1,252,065 347 (103,639)1,148,773 
Total available-for-sale5,930,103 1,085 (623,881)5,307,307 
Held-to-maturity
U.S. government and federal agency846,046 — (83,796)762,250 
State and local governments1,682,640 1,045 (248,233)1,435,452 
Residential mortgage-backed securities1,186,366 — (109,276)1,077,090 
Total held-to-maturity3,715,052 1,045 (441,305)3,274,792 
Total debt securities$9,645,155 2,130 (1,065,186)8,582,099 
Amortized cost and fair value of debt securities by contractual maturity
The following table presents the amortized cost and fair value of available-for-sale and held-to-maturity debt securities by contractual maturity at December 31, 2023. Actual maturities may differ from expected or contractual maturities since some issuers have the right to prepay obligations with or without prepayment penalties.
 December 31, 2023
 Available-for-SaleHeld-to-Maturity
(Dollars in thousands)Amortized CostFair ValueAmortized CostFair Value
Due within one year$3,632 3,653 5,233 5,213 
Due after one year through five years857,022 804,203 927,824 860,978 
Due after five years through ten years36,144 35,010 184,556 177,537 
Due after ten years39,110 36,885 1,385,660 1,215,724 
935,908 879,751 2,503,273 2,259,452 
Mortgage-backed securities 1
4,347,345 3,905,968 999,138 920,742 
Total$5,283,253 4,785,719 3,502,411 3,180,194 
______________________________
1 Mortgage-backed securities, which have prepayment provisions, are not assigned to maturity categories due to fluctuations in their prepayment speeds.
Proceeds from sales and calls of debt securities and associated gains and losses
Proceeds from sales and calls of debt securities and the associated gains and losses that have been included in earnings are listed below:
 Years ended
(Dollars in thousands)December 31,
2023		December 31,
2022		December 31,
2021
Available-for-sale
Proceeds from sales and calls of debt securities$31,944 428,225 188,431 
Gross realized gains 1
145 3,357 984 
Gross realized losses 1
(176)(2,021)(194)
Held-to-maturity
Proceeds from calls of debt securities18,125 28,210 48,475 
Gross realized gains 1
10 64 3 
Gross realized losses 1
(193)(780)(1,431)
______________________________
1 The gain or loss on the sale or call of each debt security is determined by the specific identification method.
Summary of debt securities with an unrealized loss position
The following table summarizes available-for-sale debt securities that were in an unrealized loss position for which an ACL has not been recorded, based on the length of time the individual securities have been in an unrealized loss position. The number of available-for-sale debt securities in an unrealized position is also disclosed.
 
 December 31, 2023
 Number
of
Securities		Less than 12 Months12 Months or MoreTotal
(Dollars in thousands)Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss
Available-for-sale
U.S. government and federal agency
57 $3,702 (56)448,909 (29,613)452,611 (29,669)
U.S. government sponsored enterprises
14 — — 299,220 (22,774)299,220 (22,774)
State and local governments85 3,039 (2)64,645 (3,271)67,684 (3,273)
Corporate bonds4 — — 23,262 (756)23,262 (756)
Residential mortgage-backed securities402 1,430 (44)2,809,482 (355,289)2,810,912 (355,333)
Commercial mortgage-backed securities151 21,232 (268)1,034,183 (86,302)1,055,415 (86,570)
Total available-for-sale
713 $29,403 (370)4,679,701 (498,005)4,709,104 (498,375)

 December 31, 2022
 Number
of
Securities		Less than 12 Months12 Months or MoreTotal
(Dollars in thousands)Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss
Available-for-sale
U.S. government and federal agency
56 $4,150 (64)435,375 (42,552)439,525 (42,616)
U.S. government sponsored enterprises
14 — — 287,364 (32,793)287,364 (32,793)
State and local governments121 71,512 (2,109)20,753 (2,640)92,265 (4,749)
Corporate bonds5 25,146 (992)— — 25,146 (992)
Residential mortgage-backed securities441 301,548 (24,581)2,965,512 (414,511)3,267,060 (439,092)
Commercial mortgage-backed securities157 673,102 (41,984)435,176 (61,655)1,108,278 (103,639)
Total available-for-sale
794 $1,075,458 (69,730)4,144,180 (554,151)5,219,638 (623,881)
Credit quality indictors for HTM debt securities
Allowance for Credit Losses - Held-To-Maturity Debt Securities
The Company measured expected credit losses on held-to-maturity debt securities on a collective basis by major security type and NRSRO credit ratings, which is the Company’s primary credit quality indicator for state and local government securities. The estimate of expected credit losses considered historical credit loss information that was adjusted for current conditions as well as reasonable and supportable forecasts. The following table summarizes the amortized cost of held-to-maturity municipal bonds aggregated by NRSRO credit rating:
(Dollars in thousands)December 31,
2023		December 31,
2022
Municipal bonds held-to-maturity
S&P: AAA / Moody’s: Aaa
$427,918 430,542 
S&P: AA+, AA, AA- / Moody’s: Aa1, Aa2, Aa3
1,182,894 1,206,441 
S&P: A+, A, A- / Moody’s: A1, A2, A3
37,742 37,162 
Not rated by either entity
1,446 8,495 
Total municipal bonds held-to-maturity
$1,650,000 1,682,640