The following tables summarize the amortized cost and fair value of available-for-sale securities by major type:


	At March 31, 2024
(In thousands)	Amortized Cost (1)		Unrealized Gains		Unrealized Losses		Fair Value

Government agency debentures	$	302,261	$	—	$	(39,773)	$	262,488
Municipal bonds and notes	1,257,643		23		(50,316)		1,207,350
Agency CMO	50,422		—		(4,186)		46,236
Agency MBS	3,629,162		19,414		(284,162)		3,364,414
Agency CMBS	2,619,079		12,555		(310,404)		2,321,230
CMBS	752,086		32		(20,267)		731,851

Corporate debt	693,093		142		(76,026)		617,209
Private label MBS	46,044		—		(4,736)		41,308
Other	9,836		—		(781)		9,055
Total available-for-sale	$	9,359,626	$	32,166	$	(790,651)	$	8,601,141

	At December 31, 2023
(In thousands)	Amortized Cost (1)		Unrealized Gains		Unrealized Losses		Fair Value

Government agency debentures	$	302,212	$	—	$	(37,579)	$	264,633
Municipal bonds and notes	1,626,126		8		(52,901)		1,573,233
Agency CMO	52,994		—		(4,053)		48,941
Agency MBS	3,568,140		32,461		(253,503)		3,347,098
Agency CMBS	2,569,438		18,204		(299,571)		2,288,071
CMBS	788,478		—		(24,729)		763,749

Corporate debt	704,569		—		(82,414)		622,155
Private label MBS	46,635		—		(3,827)		42,808
Other	9,830		—		(789)		9,041
Total available-for-sale	$	9,668,422	$	50,673	$	(759,366)	$	8,959,729

(1)Accrued interest receivable on available-for-sale securities of $43.6 million and $42.5 million at March 31, 2024, and December 31, 2023, respectively, is excluded from amortized cost and is included in Accrued interest receivable and other assets on the accompanying Condensed Consolidated Balance Sheets.


	At March 31, 2024
	Less Than 12 Months				12 Months or More				Total
(Dollars in thousands)	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Number of Holdings	Fair Value		Unrealized Losses

Government agency debentures	$	—	$	—	$	262,488	$	(39,773)	19	$	262,488	$	(39,773)
Municipal bonds and notes	16,078		(179)		1,180,389		(50,137)		346	1,196,467		(50,316)
Agency CMO	—		—		46,236		(4,186)		36	46,236		(4,186)
Agency MBS	187,534		(892)		1,930,864		(283,270)		462	2,118,398		(284,162)
Agency CMBS	424,108		(16,031)		1,356,358		(294,373)		148	1,780,466		(310,404)
CMBS	34,913		(87)		674,009		(20,180)		40	708,922		(20,267)

Corporate debt	—		—		612,760		(76,026)		89	612,760		(76,026)
Private label MBS	—		—		41,308		(4,736)		3	41,308		(4,736)
Other	—		—		9,055		(781)		2	9,055		(781)
Total	$	662,633	$	(17,189)	$	6,113,467	$	(773,462)	1,145	$	6,776,100	$	(790,651)


	At December 31, 2023
	Less Than Twelve Months				Twelve Months or Longer				Total
(Dollars in thousands)	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Number of Holdings	Fair Value		Unrealized Losses

Government agency debentures	$	—	$	—	$	264,633	$	(37,579)	19	$	264,633	$	(37,579)
Municipal bonds and notes	18,066		(124)		1,536,656		(52,777)		386	1,554,722		(52,901)
Agency CMO	—		—		48,941		(4,053)		36	48,941		(4,053)
Agency MBS	71,187		(272)		1,945,221		(253,231)		457	2,016,408		(253,503)
Agency CMBS	430,070		(16,137)		1,314,681		(283,434)		145	1,744,751		(299,571)
CMBS	43,844		(856)		719,905		(23,873)		42	763,749		(24,729)

Corporate debt	4,278		(27)		617,877		(82,387)		91	622,155		(82,414)
Private label MBS	—		—		42,808		(3,827)		3	42,808		(3,827)
Other	—		—		9,041		(789)		2	9,041		(789)
Total	$	567,445	$	(17,416)	$	6,499,763	$	(741,950)	1,181	$	7,067,208	$	(759,366)

The $31.3 million increase in gross unrealized losses from December 31, 2023, to March 31, 2024, is primarily due to higher interest rates. The Company assesses each available-for-sale security that is in an unrealized loss position to determine whether the decline in fair value below the amortized cost basis is a result of a credit loss or other factors. At both March 31, 2024, and December 31, 2023, no ACL was recorded on available-for-sale securities as each of the securities in the Company’s portfolio are investment grade, current as to principal and interest, and their price changes are consistent with interest and credit spreads when adjusting for convexity, rating, and industry differences.

As of March 31, 2024, based on current market conditions and the Company’s targeted balance sheet composition strategy, the Company intends to hold its available-for-sale securities with unrealized loss positions through the anticipated recovery period. The issuers of these available-for-sale securities have not, to the Company’s knowledge, established any cause for default. Market prices are expected to approach par as the securities approach maturity.

The following table summarizes the amortized cost and fair value of available-for-sale securities by contractual maturity:


	At March 31, 2024
(In thousands)	Amortized Cost		Fair Value
Maturing within 1 year	$	32,356	$	31,621
After 1 year through 5 years	597,123		565,463
After 5 through 10 years	1,271,850		1,173,728
After 10 years	7,458,297		6,830,329
Total available-for-sale	$	9,359,626	$	8,601,141

Available-for-sale securities that are not due at a single maturity date have been categorized based on the maturity date of the underlying collateral. Actual principal cash flows may differ from this categorization as borrowers have the right to repay their obligations with or without prepayment penalties.

For the three months ended March 31, 2024, and 2023, net realized losses on sale of available-for-sale securities were $12.4 million and $20.5 million, respectively. Because $2.6 million and $3.8 million of the gross realized losses for the three months ended March 31, 2024 and 2023, respectively, were attributed to a decline in credit quality, those portions have been included in the Provision for credit losses on the accompanying Condensed Consolidated Statements of Income. The amounts of $9.8 million and $16.7 million for the three months ended March 31, 2024, and 2023, respectively, that are included in
non-interest income reflect the portion of the net realized loss that was not attributed to a decline in credit quality.


	At March 31, 2024
(In thousands)	Amortized Cost (1)		Unrealized Gains		Unrealized Losses		Fair Value		Allowance		Net Carrying Value
Agency CMO	$	22,489	$	—	$	(1,938)	$	20,551	$	—	$	22,489
Agency MBS	2,618,783		1,429		(327,351)		2,292,861		—		2,618,783
Agency CMBS	4,028,799		11,580		(540,641)		3,499,738		—		4,028,799
Municipal bonds and notes	910,422		683		(34,953)		876,152		184		910,238
CMBS	99,582		—		(6,017)		93,565		—		99,582
Total held-to-maturity	$	7,680,075	$	13,692	$	(910,900)	$	6,782,867	$	184	$	7,679,891


	At December 31, 2023
(In thousands)	Amortized Cost (1)		Unrealized Gains		Unrealized Losses		Fair Value		Allowance		Net Carrying Value
Agency CMO	$	23,470	$	—	$	(1,728)	$	21,742	$	—	$	23,470
Agency MBS	2,409,521		1,141		(284,776)		2,125,886		—		2,409,521
Agency CMBS	3,625,627		18,586		(514,534)		3,129,679		—		3,625,627
Municipal bonds and notes	916,104		2,440		(24,877)		893,667		209		915,895
CMBS	100,075		—		(6,426)		93,649		—		100,075
Total held-to-maturity	$	7,074,797	$	22,167	$	(832,341)	$	6,264,623	$	209	$	7,074,588

(1)Accrued interest receivable on held-to-maturity securities of $22.6 million and $24.9 million at March 31, 2024, and December 31, 2023, respectively, is excluded from amortized cost and is included in Accrued interest receivable and other assets on the accompanying Condensed Consolidated Balance Sheets.

An ACL on held-to-maturity securities is recorded for certain Municipal bonds and notes to account for expected lifetime credit losses. Agency securities represent obligations issued by a U.S. government-sponsored enterprise or other federally-related entity and are either explicitly or implicitly guaranteed, and therefore, assumed to be zero loss. Held-to-maturity securities with gross unrealized losses and no ACL are considered to be high credit quality, and therefore, zero credit loss has been recorded.

The following table summarizes the activity in the ACL on held-to-maturity securities:


	Three months ended March 31,
(In thousands)	2024		2023
Balance, beginning of period	$	209	$	182

(Benefit) provision for credit losses	(25)		100
Balance, end of period	$	184	$	282

The following table summarizes the amortized cost and fair value of held-to-maturity securities by contractual maturity:


	At March 31, 2024
(In thousands)	Amortized Cost		Fair Value
Maturing within 1 year	$	29,846	$	29,771
After 1 year through 5 years	94,598		89,326
After 5 through 10 years	299,712		288,067
After 10 years	7,255,919		6,375,703
Total held-to-maturity	$	7,680,075	$	6,782,867

Held-to-maturity securities that are not due at a single maturity date have been categorized based on the maturity date of the underlying collateral. Actual principal cash flows may differ from this categorization as borrowers have the right to prepay their obligations with or without prepayment penalties.

The Company monitors the credit quality of held-to-maturity securities through credit ratings provided by Standard & Poor’s Rating Services, Moody’s Investor Services, Fitch Ratings, Inc., Kroll Bond Rating Agency, or DBRS Inc. Credit ratings express opinions about the credit quality of a security and are updated at each quarter end. Investment grade securities are rated BBB- or higher by S&P, or Baa3 or higher by Moody’s, and are generally considered by the rating agencies and market participants to be of low credit risk. Conversely, securities rated below investment grade, which are labeled as speculative grade by the rating agencies, are considered to have distinctively higher credit risk than investment grade securities. There were no speculative grade held-to-maturity securities at either March 31, 2024, or December 31, 2023. Held-to-maturity securities that are not rated are collateralized with U.S. Treasury obligations.


	March 31, 2024
	Investment Grade
(In thousands)	Aaa		Aa1		Aa2		Aa3		A1		A3		Not Rated
Agency CMO	$	—	$	22,489	$	—	$	—	$	—	$	—	$	—
Agency MBS	—		2,618,783		—		—		—		—		—
Agency CMBS	—		4,028,799		—		—		—		—		—
Municipal bonds and notes	332,969		162,439		252,852		111,277		32,690		4,165		14,030
CMBS	99,582		—		—		—		—		—		—
Total held-to-maturity	$	432,551	$	6,832,510	$	252,852	$	111,277	$	32,690	$	4,165	$	14,030


	December 31, 2023
	Investment Grade
(In thousands)	Aaa		Aa1		Aa2		Aa3		A1		A3		Not Rated
Agency CMO	$	—	$	23,470	$	—	$	—	$	—	$	—	$	—
Agency MBS	—		2,409,521		—		—		—		—		—
Agency CMBS	—		3,625,627		—		—		—		—		—
Municipal bonds and notes	333,479		162,615		253,671		115,404		32,732		4,165		14,038
CMBS	100,075		—		—		—		—		—		—
Total held-to-maturity	$	433,554	$	6,221,233	$	253,671	$	115,404	$	32,732	$	4,165	$	14,038