INVESTMENTS (Tables)

3 Months Ended

Mar. 31, 2021

Investments, Debt and Equity Securities [Abstract]

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.

	March 31, 2021
(Dollars in thousands)	Amortized Cost		Gross Unrealized  Gain		Gross Unrealized  Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
CMO	$	564,055	$	5,604	$	15,223	$	—	$	554,436
FNMA MBS	1,997,140		35,151		34,246		—		1,998,045
FHLMC MBS	182,271		8,941		759		—		190,453
GNMA MBS	25,011		907		83		—		25,835
GSE agency notes	232,509		—		13,393		—		219,116
	$	3,000,986	$	50,603	$	63,704	$	—	$	2,987,885
Held-to-Maturity Debt Securities(1)
State and political subdivisions	$	103,027	$	4,107	$	—	$	5	$	107,129
Foreign bonds	501		—		1		—		500
	$	103,528	$	4,107	$	1	$	5	$	107,629

(1)Held-to-maturity securities transferred from available-for-sale are included in held-to-maturity at amortized cost basis at the time of transfer. The amortized cost of transferred held-to-maturity securities included net unrealized gains of $0.3 million at March 31, 2021, which are offset in Accumulated other comprehensive (loss) income. 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, 2020
(Dollars in thousands)	Amortized Cost		Gross Unrealized  Gain		Gross Unrealized  Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
CMO	$	461,819	$	9,949	$	443	$	—	$	471,325
FNMA MBS	1,544,105		55,747		882		—		1,598,970
FHLMC MBS	190,856		12,142		105		—		202,893
GNMA MBS	22,716		1,046		—		—		23,762
GSE agency notes	230,769		1,987		649		—		232,107
	$	2,450,265	$	80,871	$	2,079	$	—	$	2,529,057
Held-to-Maturity Debt Securities(1)
State and political subdivisions	$	111,246	$	4,678	$	—	$	6	$	115,918
Foreign bonds	501		2		—		—		503
	$	111,747	$	4,680	$	—	$	6	$	116,421

(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 gains of $0.4 million at December 31, 2020, which are offset in Accumulated other comprehensive (loss) income. 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.

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.

	March 31, 2021
(Dollars in thousands)	Amortized Cost		Gross Unrealized  Gain		Gross Unrealized  Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
CMO	$	564,055	$	5,604	$	15,223	$	—	$	554,436
FNMA MBS	1,997,140		35,151		34,246		—		1,998,045
FHLMC MBS	182,271		8,941		759		—		190,453
GNMA MBS	25,011		907		83		—		25,835
GSE agency notes	232,509		—		13,393		—		219,116
	$	3,000,986	$	50,603	$	63,704	$	—	$	2,987,885
Held-to-Maturity Debt Securities(1)
State and political subdivisions	$	103,027	$	4,107	$	—	$	5	$	107,129
Foreign bonds	501		—		1		—		500
	$	103,528	$	4,107	$	1	$	5	$	107,629

(1)Held-to-maturity securities transferred from available-for-sale are included in held-to-maturity at amortized cost basis at the time of transfer. The amortized cost of transferred held-to-maturity securities included net unrealized gains of $0.3 million at March 31, 2021, which are offset in Accumulated other comprehensive (loss) income. 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, 2020
(Dollars in thousands)	Amortized Cost		Gross Unrealized  Gain		Gross Unrealized  Loss		Allowance for Credit Losses		Fair Value
Available-for-Sale Debt Securities
CMO	$	461,819	$	9,949	$	443	$	—	$	471,325
FNMA MBS	1,544,105		55,747		882		—		1,598,970
FHLMC MBS	190,856		12,142		105		—		202,893
GNMA MBS	22,716		1,046		—		—		23,762
GSE agency notes	230,769		1,987		649		—		232,107
	$	2,450,265	$	80,871	$	2,079	$	—	$	2,529,057
Held-to-Maturity Debt Securities(1)
State and political subdivisions	$	111,246	$	4,678	$	—	$	6	$	115,918
Foreign bonds	501		2		—		—		503
	$	111,747	$	4,680	$	—	$	6	$	116,421

(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 gains of $0.4 million at December 31, 2020, which are offset in Accumulated other comprehensive (loss) income. 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 months ended March 31, 2021 and 2020:

	Three months ended March 31,
(Dollars in thousands)	2021		2020
Allowance for credit losses:
Beginning balance	$	6	$	—
Impact of adoption ASC 326	—		8
Provision for credit losses	(1)		—
Charge-offs, net	—		—
Ending balance	$	5	$	8

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

The scheduled maturities of available-for-sale debt securities at March 31, 2021 and December 31, 2020 are presented in the table below:

	Available-for-Sale
	Amortized		Fair
(Dollars in thousands)	Cost		Value
March 31, 2021 (1)
Within one year	$	—	$	—
After one year but within five years	57,993		60,781
After five years but within ten years	233,748		235,304
After ten years	2,709,245		2,691,800
	$	3,000,986	$	2,987,885
December 31, 2020 (1)
Within one year	$	—	$	—
After one year but within five years	37,852		39,985
After five years but within ten years	239,845		251,874
After ten years	2,172,568		2,237,198
	$	2,450,265	$	2,529,057

(1)Actual maturities could differ from contractual maturities.

The scheduled maturities of held-to-maturity debt securities at March 31, 2021 and December 31, 2020 are presented in the table below:

	Held-to-Maturity
	Amortized		Fair
(Dollars in thousands)	Cost		Value
March 31, 2021 (1)
Within one year	$	1,135	$	1,140
After one year but within five years	971		985
After five years but within ten years	41,075		42,558
After ten years	60,347		62,946
	$	103,528	$	107,629
December 31, 2020 (1)
Within one year	$	1,144	$	1,154
After one year but within five years	972		990
After five years but within ten years	35,967		37,317
After ten years	73,664		76,960
	$	111,747	$	116,421

(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 and an allowance has not been recorded, 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 March 31, 2021.

	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	$	288,625	$	15,223	$	—	$	—	$	288,625	$	15,223
FNMA MBS	1,241,199		34,243		4,330		3		1,245,529		34,246
FHLMC MBS	17,936		759		—		—		17,936		759
GNMA MBS	4,969		83		—		—		4,969		83
GSE agency notes	219,116		13,393		—		—		219,116		13,393
	$	1,771,845	$	63,701	$	4,330	$	3	$	1,776,175	$	63,704
Held-to-maturity debt securities:
Foreign bonds	$	500	$	1	$	—	$	—	$	500	$	1
	$	500	$	1	$	—	$	—	$	500	$	1

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, 2020.

	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	$	183,983	$	443	$	—	$	—	$	183,983	$	443
FNMA MBS	289,338		879		4,355		3		293,693		882
FHLMC MBS	5,191		105		—		—		5,191		105
GSE agency notes	101,016		649		—		—		101,016		649
	$	579,528	$	2,076	$	4,355	$	3	$	583,883	$	2,079

Held-to-maturity, Credit Quality Indicator

The following table summarizes the amortized cost of debt securities held-to-maturity as of March 31, 2021, aggregated by credit quality indicator:

(Dollars in thousands)	State and political subdivisions		Foreign bonds
A+ rated or higher	$	102,901	$	501
Not rated	126		—
Ending balance	$	103,027	$	501

The following table summarizes the amortized cost of debt securities held-to-maturity as of December 31, 2020, aggregated by credit quality indicator:

(Dollars in thousands)	State and political subdivisions		Foreign bonds
A+ rated or higher	$	110,959	$	501
Not rated	287		—
Ending balance	$	111,246	$	501