Debt Securities (Tables)

3 Months Ended

Mar. 31, 2021

Investments, Debt and Equity Securities [Abstract]

Amortized cost, gross unrealized gains and losses and 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:

	March 31, 2021
(Dollars in thousands)	Amortized Cost		Gross Unrealized Gains	Gross Unrealized Losses	Fair Value
Available-for-sale
U.S. government and federal agency	$	36,122	231	(490)	35,863
U.S. government sponsored enterprises	5,231		22	—	5,253
State and local governments	914,572		64,593	(513)	978,652
Corporate bonds	278,914		9,952	(85)	288,781
Residential mortgage-backed securities	3,369,683		16,420	(22,476)	3,363,627
Commercial mortgage-backed securities	1,145,668		39,958	(4,487)	1,181,139
Total available-for-sale	$	5,750,190	131,176	(28,051)	5,853,315
Held-to-maturity
State and local governments	$	588,751	10,937	(728)	598,960
Total held-to-maturity	$	588,751	10,937	(728)	598,960

	December 31, 2020
(Dollars in thousands)	Amortized Cost		Gross Unrealized Gains	Gross Unrealized Losses	Fair Value
Available-for-sale
U.S. government and federal agency	$	38,568	287	(267)	38,588
U.S. government sponsored enterprises	9,747		34	—	9,781
State and local governments	1,321,763		94,974	(54)	1,416,683
Corporate bonds	336,867		12,239	(8)	349,098
Residential mortgage-backed securities	2,261,463		27,631	(4)	2,289,090
Commercial mortgage-backed securities	1,177,458		57,575	(459)	1,234,574
Total available-for-sale	$	5,145,866	192,740	(792)	5,337,814
Held-to-maturity
State and local governments	$	189,836	13,380	—	203,216
Total held-to-maturity	$	189,836	13,380	—	203,216

Amortized cost and fair value of 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 March 31, 2021. Actual maturities may differ from expected or contractual maturities since some issuers have the right to prepay obligations with or without prepayment penalties.

	March 31, 2021
	Available-for-Sale			Held-to-Maturity
(Dollars in thousands)	Amortized Cost		Fair Value	Amortized Cost	Fair Value
Due within one year	$	91,067	92,128	482	486
Due after one year through five years	237,676		248,388	23,431	24,883
Due after five years through ten years	247,854		257,032	66,883	71,375
Due after ten years	658,242		711,001	497,955	502,216
	1,234,839		1,308,549	588,751	598,960
Mortgage-backed securities 1	4,515,351		4,544,766	—	—
Total	$	5,750,190	5,853,315	588,751	598,960

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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 the 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:

	Three Months ended
(Dollars in thousands)	March 31, 2021	March 31, 2020
Available-for-sale
Proceeds from sales and calls of debt securities	54,336	77,073
Gross realized gains 1	369	962
Gross realized losses 1	(43)	(100)
Held-to-maturity
Proceeds from calls of debt securities	4,130	20,250
Gross realized gains 1	—	1
Gross realized losses 1	(42)	—

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1 The gain or loss on the sale or call of each debt security is determined by the specific identification method.

AFS debt securities in an unrealized loss position with no ACL recorded

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.

	March 31, 2021
	Number of Securities	Less than 12 Months			12 Months or More		Total
(Dollars in thousands)		Fair Value		Unrealized Loss	Fair Value	Unrealized Loss	Fair Value	Unrealized Loss
Available-for-sale
U.S. government and federal agency	24	$	17,857	(343)	2,361	(147)	20,218	(490)
State and local governments	57	36,748		(480)	816	(33)	37,564	(513)
Corporate bonds	1	3,915		(85)	—	—	3,915	(85)
Residential mortgage-backed securities	80	2,158,485		(22,475)	27	(1)	2,158,512	(22,476)
Commercial mortgage-backed securities	13	237,514		(4,487)	—	—	237,514	(4,487)
Total available-for-sale	175	$	2,454,519	(27,870)	3,204	(181)	2,457,723	(28,051)
Held-to-maturity
State and local governments	65	$	217,131	(728)	—	—	217,131	(728)
Total held-to-maturity	65	$	217,131	(728)	—	—	217,131	(728)

	December 31, 2020
	Number of Securities	Less than 12 Months			12 Months or More		Total
(Dollars in thousands)		Fair Value		Unrealized Loss	Fair Value	Unrealized Loss	Fair Value	Unrealized Loss
Available-for-sale
U.S. government and federal agency	18	$	13,814	(258)	726	(9)	14,540	(267)
State and local governments	6	3,121		(54)	—	—	3,121	(54)
Corporate bonds	3	5,500		(8)	—	—	5,500	(8)
Residential mortgage-backed securities	14	2,354		(4)	27	—	2,381	(4)
Commercial mortgage-backed securities	5	120,741		(459)	—	—	120,741	(459)
Total available-for-sale	46	$	145,530	(783)	753	(9)	146,283	(792)

Credit quality indictors for HTM debt securities

The following table summarizes the amortized cost of held-to-maturity debt securities aggregated by NRSRO credit rating:

(Dollars in thousands)	March 31, 2021		December 31, 2020
Held-to-maturity
S&P: AAA / Moody’s: Aaa	$	91,831	39,022
S&P: AA+, AA, AA- / Moody’s: Aa1, Aa2, Aa3	471,159		123,664
S&P: A+, A, A- / Moody’s: A1, A2, A3	25,761		27,150
Total held-to-maturity	$	588,751	189,836