INVESTMENT SECURITIES (Tables)

9 Months Ended

Sep. 30, 2023

Investments, Debt and Equity Securities [Abstract]

Schedule of Debt Securities, Available-for-sale

The following tables detail the amortized cost, allowance for credit losses and the estimated fair value of the Company's investments in available-for-sale and held-to-maturity debt securities. None of the Company's investments in debt securities are classified as trading.

	September 30, 2023
(Dollars in thousands)	Amortized Cost		Gross Unrealized  Gain		Gross Unrealized  Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
Collateralized mortgage obligation (CMO)	$	573,393	$	—	$	118,729	$	—	$	454,664
Fannie Mae (FNMA) mortgage-backed securities (MBS)	3,608,095		—		694,683		—		2,913,412
Freddie Mac (FHLMC) MBS	128,999		—		18,280		—		110,719
Ginnie Mae (GNMA) MBS	47,621		—		4,870		—		42,751
Government-sponsored enterprises (GSE) agency notes	226,082		—		56,087		—		169,995
	$	4,584,190	$	—	$	892,649	$	—	$	3,691,541
Held-to-Maturity Debt Securities(1)
FNMA MBS	$	881,844	$	—	$	129,388	$	—	$	752,456
State and political subdivisions	187,035		24		11,827		8		175,224
	$	1,068,879	$	24	$	141,215	$	8	$	927,680

(1)Held-to-maturity securities transferred from available-for-sale are included in held-to-maturity at fair value basis at the time of transfer. The amortized cost of transferred held-to-maturity securities included net unrealized losses of $125.8 million at September 30, 2023, which are offset in Accumulated other comprehensive loss. At the time of transfer, there was no allowance for credit loss on the available-for-sale securities. Subsequent to transfer, the securities were evaluated for credit loss.

	December 31, 2022
(Dollars in thousands)	Amortized Cost		Gross Unrealized  Gain		Gross Unrealized  Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
CMO	$	608,834	$	—	$	102,454	$	—	$	506,380
FNMA MBS	3,823,036		—		572,778		—		3,250,258
FHLMC MBS	135,554		—		13,555		—		121,999
GNMA MBS	39,116		—		2,978		—		36,138
GSE agency notes	228,010		—		49,725		—		178,285
	$	4,834,550	$	—	$	741,490	$	—	$	4,093,060
Held-to-Maturity Debt Securities(1)
FNMA MBS	$	909,498	$	—	$	68,677	$	—	$	840,821
State and political subdivisions	201,631		532		3,372		10		198,781
Foreign bonds	500		2		—		—		502
	$	1,111,629	$	534	$	72,049	$	10	$	1,040,104

(1)Held-to-maturity securities transferred from available-for-sale are included in held-to-maturity at fair value at the time of transfer. The amortized cost of transferred held-to-maturity securities included net unrealized losses of $142.8 million at December 31, 2022, which are offset in Accumulated other comprehensive loss. At the time of transfer, there was no allowance for credit loss on the available-for-sale securities. Subsequent to transfer, the securities were evaluated for credit loss.

Schedule of Debt Securities, Held-to-maturity

The following tables detail the amortized cost, allowance for credit losses and the estimated fair value of the Company's investments in available-for-sale and held-to-maturity debt securities. None of the Company's investments in debt securities are classified as trading.

	September 30, 2023
(Dollars in thousands)	Amortized Cost		Gross Unrealized  Gain		Gross Unrealized  Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
Collateralized mortgage obligation (CMO)	$	573,393	$	—	$	118,729	$	—	$	454,664
Fannie Mae (FNMA) mortgage-backed securities (MBS)	3,608,095		—		694,683		—		2,913,412
Freddie Mac (FHLMC) MBS	128,999		—		18,280		—		110,719
Ginnie Mae (GNMA) MBS	47,621		—		4,870		—		42,751
Government-sponsored enterprises (GSE) agency notes	226,082		—		56,087		—		169,995
	$	4,584,190	$	—	$	892,649	$	—	$	3,691,541
Held-to-Maturity Debt Securities(1)
FNMA MBS	$	881,844	$	—	$	129,388	$	—	$	752,456
State and political subdivisions	187,035		24		11,827		8		175,224
	$	1,068,879	$	24	$	141,215	$	8	$	927,680

(1)Held-to-maturity securities transferred from available-for-sale are included in held-to-maturity at fair value basis at the time of transfer. The amortized cost of transferred held-to-maturity securities included net unrealized losses of $125.8 million at September 30, 2023, which are offset in Accumulated other comprehensive loss. At the time of transfer, there was no allowance for credit loss on the available-for-sale securities. Subsequent to transfer, the securities were evaluated for credit loss.

	December 31, 2022
(Dollars in thousands)	Amortized Cost		Gross Unrealized  Gain		Gross Unrealized  Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
CMO	$	608,834	$	—	$	102,454	$	—	$	506,380
FNMA MBS	3,823,036		—		572,778		—		3,250,258
FHLMC MBS	135,554		—		13,555		—		121,999
GNMA MBS	39,116		—		2,978		—		36,138
GSE agency notes	228,010		—		49,725		—		178,285
	$	4,834,550	$	—	$	741,490	$	—	$	4,093,060
Held-to-Maturity Debt Securities(1)
FNMA MBS	$	909,498	$	—	$	68,677	$	—	$	840,821
State and political subdivisions	201,631		532		3,372		10		198,781
Foreign bonds	500		2		—		—		502
	$	1,111,629	$	534	$	72,049	$	10	$	1,040,104

(1)Held-to-maturity securities transferred from available-for-sale are included in held-to-maturity at fair value at the time of transfer. The amortized cost of transferred held-to-maturity securities included net unrealized losses of $142.8 million at December 31, 2022, which are offset in Accumulated other comprehensive loss. At the time of transfer, there was no allowance for credit loss on the available-for-sale securities. Subsequent to transfer, the securities were evaluated for credit loss.

The following table presents the activity in the allowance for credit losses for state and political subdivisions debt securities for the three and nine months ended September 30, 2023 and 2022:

	Three months ended September 30,				Nine months ended September 30,
(Dollars in thousands)	2023		2022		2023		2022
Allowance for credit losses:
Beginning balance	$	8	$	6	$	10	$	4
Provision for (recovery of) credit losses	—		4		(2)		6
Ending balance	$	8	$	10	$	8	$	10

Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity

The scheduled maturities of available-for-sale debt securities at September 30, 2023 and December 31, 2022 are presented in the table below:

	Available-for-Sale
	Amortized		Fair
(Dollars in thousands)	Cost		Value
September 30, 2023 (1)
Within one year	$	—	$	—
After one year but within five years	87,314		81,548
After five years but within ten years	577,578		465,454
After ten years	3,919,298		3,144,539
	$	4,584,190	$	3,691,541
December 31, 2022 (1)
Within one year	$	—	$	—
After one year but within five years	83,014		77,499
After five years but within ten years	465,777		398,607
After ten years	4,285,759		3,616,954
	$	4,834,550	$	4,093,060

(1)Actual maturities could differ from contractual maturities.

The scheduled maturities of held-to-maturity debt securities at September 30, 2023 and December 31, 2022 are presented in the table below:

	Held-to-Maturity
	Amortized		Fair
(Dollars in thousands)	Cost		Value
September 30, 2023 (1)
Within one year	$	—	$	—
After one year but within five years	9,793		9,373
After five years but within ten years	45,306		42,615
After ten years	1,013,780		875,692
	$	1,068,879	$	927,680
December 31, 2022 (1)
Within one year	$	731	$	732
After one year but within five years	9,530		9,476
After five years but within ten years	46,170		45,944
After ten years	1,055,198		983,952
	$	1,111,629	$	1,040,104

(1)Actual maturities could differ from contractual maturities.

Schedule of Investment Securities' Gross Unrealized Losses and Fair Value by Investment Category

For debt securities in an unrealized loss position, the table below shows the gross unrealized losses and fair value by investment category and length of time that individual debt securities were in a continuous unrealized loss position at September 30, 2023.

	Duration of Unrealized Loss Position
	Less than 12 months				12 months or longer				Total
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
(Dollars in thousands)	Value		Loss		Value		Loss		Value		Loss
Available-for-sale debt securities:
CMO	$	—	$	—	$	454,664	$	118,729	$	454,664	$	118,729
FNMA MBS	8,792		467		2,904,620		694,216		2,913,412		694,683
FHLMC MBS	7		—		110,712		18,280		110,719		18,280
GNMA MBS	11,421		654		31,330		4,216		42,751		4,870
GSE agency notes	—		—		169,995		56,087		169,995		56,087
	$	20,220	$	1,121	$	3,671,321	$	891,528	$	3,691,541	$	892,649

For debt securities in an unrealized loss position, the table below shows the gross unrealized losses and fair value by investment category and length of time that individual debt securities were in a continuous unrealized loss position at December 31, 2022.

	Duration of Unrealized Loss Position
	Less than 12 months				12 months or longer				Total
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
(Dollars in thousands)	Value		Loss		Value		Loss		Value		Loss
Available-for-sale debt securities:
CMO	$	158,449	$	13,855	$	347,931	$	88,599	$	506,380	$	102,454
FNMA MBS	1,237,560		145,752		2,012,698		427,026		3,250,258		572,778
FHLMC MBS	102,321		9,268		19,671		4,287		121,992		13,555
GNMA MBS	32,076		2,265		4,030		713		36,106		2,978
GSE agency notes	—		—		178,285		49,725		178,285		49,725
	$	1,530,406	$	171,140	$	2,562,615	$	570,350	$	4,093,021	$	741,490

Schedule of Held-to-Maturity Credit Quality Indicators

The following table summarizes the amortized cost of debt securities held-to-maturity as of September 30, 2023, aggregated by credit quality indicator:

(Dollars in thousands)	FNMA MBS		State and political subdivisions
A+ rated or higher	$	—	$	187,035
Not rated	881,844		—
Ending balance	$	881,844	$	187,035

The following table summarizes the amortized cost of debt securities held-to-maturity as of December 31, 2022, aggregated by credit quality indicator:

(Dollars in thousands)	FNMA MBS		State and political subdivisions		Foreign bonds
A+ rated or higher	$	—	$	201,631	$	500
Not rated	909,498		—		—
Ending balance	$	909,498	$	201,631	$	500