Securities (Tables)

3 Months Ended

Mar. 31, 2024

Investments, Debt and Equity Securities [Abstract]

Summary of Amortized Cost and Estimated Fair Values of Debt Securities Held for Investment

The following table summarizes the amortized cost and estimated fair values of debt securities held for investment:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
March 31, 2024	(Dollars in thousands)
Mortgage backed securities (1)	$	4	$	—	$	—	$	4
States and political subdivisions		625		—		—		625
Other securities		500		—		—		500
Total	$	1,129	$	—	$	—	$	1,129
December 31, 2023
Mortgage backed securities (1)	$	5	$	—	$	—	$	5
States and political subdivisions		685		—		—		685
Other securities		500		—		—		500
Total	$	1,190	$	—	$	—	$	1,190

Summary of Amortized Cost and Estimated Fair Values of Debt Securities Available for Sale

The following table summarizes the amortized cost and estimated fair values of debt securities available for sale:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
March 31, 2024	(Dollars in thousands)
U.S. treasuries	$	1,545,588	$	—	$	(65,207	)	$	1,480,381
U.S. federal agencies		10,729		119		(4	)		10,844
Mortgage backed securities (1)		16,049		9		(1,795	)		14,263
States and political subdivisions		10,100		7		(137	)		9,970
Asset backed securities		11,440		—		(188	)		11,252
Other securities		8,163		—		(1,351	)		6,812
Total	$	1,602,069	$	135	$	(68,682	)	$	1,533,522
December 31, 2023
U.S. treasuries	$	1,560,265	$	415	$	(62,635	)	$	1,498,045
U.S. federal agencies		11,631		142		(3	)		11,770
Mortgage backed securities (1)		16,459		13		(1,677	)		14,795
States and political subdivisions		10,108		16		(114	)		10,010
Asset backed securities		12,794		—		(282	)		12,512
Other securities		8,163		—		(1,390	)		6,773
Total	$	1,619,420	$	586	$	(66,101	)	$	1,553,905

(1) Primarily consists of FHLMC, FNMA, GNMA and mortgage backed securities through U.S. agencies.

Maturity of Debt Securities

The maturities of debt securities held for investment and available for sale are summarized in the following table using contractual maturities. Actual maturities may differ from contractual maturities due to obligations that are called or prepaid. For purposes of the maturity table, mortgage-backed securities, which are not due at a single maturity date, have been presented at their contractual maturity.

	March 31, 2024				December 31, 2023
	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
	(Dollars in thousands)
Held for Investment
Contractual maturity of debt securities:
Within one year	$	351	$	351	$	350	$	350
After one year but within five years		778		778		840		840
After five years but within ten years		—		—		—		—
After ten years		—		—		—		—
Total	$	1,129	$	1,129	$	1,190	$	1,190
Available for Sale
Contractual maturity of debt securities:
Within one year	$	389,115	$	382,870	$	348,318	$	341,645
After one year but within five years		1,167,880		1,108,898		1,223,529		1,167,973
After five years but within ten years		10,278		8,848		10,331		8,851
After ten years		34,796		32,906		37,242		35,436
Total debt securities	$	1,602,069	$	1,533,522	$	1,619,420	$	1,553,905

Company's Book Value of Pledged Debt Securities

The following table is a summary of the Company’s book value of securities that were pledged as collateral for public funds on deposit, repurchase agreements and for other purposes as required or permitted by law:

	March 31, 2024		December 31, 2023
	(Dollars in thousands)
Book value of pledged securities	$	908,521	$	591,324

Summary of Debt Securities with Unrealized Losses, Segregated by Duration of Unrealized Loss

The following table summarizes debt securities with unrealized losses, segregated by the duration of the unrealized loss, at March 31, 2024 and December 31, 2023 respectively:

			Less than 12 Months				More than 12 Months				Total
	Number of investments		Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses
			(Dollars in thousands)
March 31, 2024
Available for Sale
U.S. treasuries		67	$	90,252	$	816	$	1,370,129	$	64,391	$	1,460,381	$	65,207
U.S. federal agencies		3		1,057		4		—		—		1,057		4
Mortgage backed securities		71		1,191		6		12,767		1,789		13,958		1,795
States and political subdivisions		9		867		3		2,128		134		2,995		137
Asset backed securities		1		—		—		11,252		188		11,252		188
Other securities		3		—		—		6,812		1,351		6,812		1,351
Total		154	$	93,367	$	829	$	1,403,088	$	67,853	$	1,496,455	$	68,682
December 31, 2023
Available for Sale
U.S. treasuries		68	$	4,838	$	90	$	1,401,669	$	62,545	$	1,406,507	$	62,635
U.S. federal agencies		3		1,100		3		—		—		1,100		3
Mortgage backed securities		74		80		—		13,261		1,677		13,341		1,677
States and political subdivisions		6		306		4		1,847		110		2,153		114
Asset backed securities		1		—		—		12,512		282		12,512		282
Other securities		3		—		—		6,773		1,390		6,773		1,390
Total		155	$	6,324	$	97	$	1,436,062	$	66,004	$	1,442,386	$	66,101