Investment Securities (Tables)

12 Months Ended

Dec. 31, 2020

Investments, Debt and Equity Securities [Abstract]

Schedule of Amortized Cost and Fair Values of Investment Securities

The following tables present the amortized cost and estimated fair values of investment securities, as of December 31:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
	(in thousands)
2020
Available for Sale
State and municipal securities	$	891,327	$	61,286	$	—	$	952,613
Corporate debt securities	348,391		19,445		(691)		367,145
Collateralized mortgage obligations	491,321		12,560		(115)		503,766
Residential mortgage-backed securities	373,779		4,246		(27)		377,998
Commercial mortgage-backed securities	741,172		22,384		(1,141)		762,415
Auction rate securities	101,510		—		(3,304)		98,206
Total	$	2,947,500	$	119,921	$	(5,278)	$	3,062,143
Held to Maturity
Residential mortgage-backed securities	$	278,281	$	18,576	$	—	$	296,857
Total	$	278,281	$	18,576	$	—	$	296,857
2019
Available for Sale
State and municipal securities	$	638,125	$	15,826	$	(1,024)	$	652,927
Corporate debt securities	370,401		8,490		(1,534)		377,357
Collateralized mortgage obligations	682,307		11,726		(315)		693,718
Residential mortgage-backed securities	177,183		1,078		(949)		177,312
Commercial mortgage-backed securities	489,603		6,471		(1,777)		494,297
Auction rate securities	107,410		—		(5,484)		101,926
Total	$	2,465,029	$	43,591	$	(11,083)	$	2,497,537
Held to Maturity
Residential mortgage-backed securities	$	369,841	$	13,864	$	—	$	383,705
Total	$	369,841	$	13,864	$	—	$	383,705

Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities

The amortized cost and estimated fair values of debt securities as of December 31, 2020, by contractual maturity, are shown in the following table. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.

	Available for Sale				Held to Maturity
	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
	(in thousands)
Due in one year or less	$	11,250	$	11,350	$	—	$	—
Due from one year to five years	39,069		40,717		—		—
Due from five years to ten years	327,456		345,612		—		—
Due after ten years	963,453		1,020,285		—		—
	1,341,228		1,417,964		—		—
Residential mortgage-backed securities (1)	373,779		377,998		278,281		296,857
Commercial mortgage-backed securities (1)	741,172		762,415		—		—
Collateralized mortgage obligations (1)	491,321		503,766		—		—
Total	$	2,947,500	$	3,062,143	$	278,281	$	296,857

(1)     Maturities for mortgage-backed securities and collateralized mortgage obligations are dependent upon the interest rate environment and prepayments on the underlying loans.

Summary of Gains and Losses from Equity and Debt Securities, and Losses Recognized from Other-than-Temporary Impairment

The following table presents information related to gross gains and losses on the sales of securities:

	Gross Realized Gains		Gross Realized Losses		Net Gains
	(in thousands)
2020	$	6,545	$	(3,492)	$	3,053
2019	11,554		(6,821)		4,733
2018	1,665		(1,628)		37

Gross Unrealized Losses and Fair Values of Investments by Category and Length of Time in Continuous Unrealized Loss Position

The following tables present the gross unrealized losses and estimated fair values of investments, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, as of December 31:

	Less Than 12 months					12 Months or Longer					Total
	Number of Securities	Estimated Fair Value		Unrealized Losses		Number of Securities	Estimated Fair Value		Unrealized Losses		Estimated Fair Value		Unrealized Losses
2020	(dollars in thousands)
Available for Sale
Corporate debt securities	9	$	44,528	$	(377)	1	$	6,871	$	(314)	$	51,399	$	(691)
Collateralized mortgage obligations	3	57,601		(115)		—	—		—		57,601		(115)
Residential mortgage-backed securities	1	20,124		(27)		—	—		—		20,124		(27)
Commercial mortgage-backed securities	9	144,383		(1,141)		—	—		—		144,383		(1,141)
Auction rate securities	—	—		—		162	98,206		(3,304)		98,206		(3,304)
Total available for sale	22	$	266,636	$	(1,660)	163	$	105,077	$	(3,618)	$	371,713	$	(5,278)

2019
Available for Sale
State and municipal securities	44	$	136,344	$	(1,024)	—	$	—	$	—	$	136,344	$	(1,024)
Corporate debt securities	5	30,719		(346)		8	18,759		(1,188)		49,478		(1,534)
Collateralized mortgage obligations	5	33,865		(190)		1	5,330		(125)		39,195		(315)
Residential mortgage-backed securities	5	12,247		(40)		26	127,373		(909)		139,620		(949)
Commercial mortgage-backed securities	7	121,340		(1,777)		—	—		—		121,340		(1,777)
Auction rate securities	—	—		—		177	101,926		(5,484)		101,926		(5,484)
Total available for sale	66	$	334,515	$	(3,377)	212	$	253,388	$	(7,706)	$	587,903	$	(11,083)